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Magnetic Field Observations from the GOES-R Series
C H A P T E R
21 Magnetic Field Observations from the GOES-R Series Paul T.M. Loto’aniu*,†, Samuel Califf*,†, Robert J. Redmon†, Howard J. Singer‡ *Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado-Boulder, Boulder, CO, United States, †NOAA National Centers for Environmental Information (NCEI), Boulder, CO, United States, ‡Space Weather Prediction Center, National Oceanic and Atmospheric Administration, Boulder, CO, United States
21.1 INTRODUCTION The Geostationary Operational Environmental Satellites (GOES)-R Series Magnetometer (MAG) continues the decades-long observations of Earth’s magnetic field at geostationary orbit by the National Oceanic and Atmospheric Administration (NOAA) GOES. These magnetic observations are among the most used space physics data sets in the world and are important for both space weather operations and for developing a better understanding of important space weather processes in the near-Earth space environment. The MAG consists of two fluxgate magnetometers (FGM) mounted on a boom. As with previous GOES, MAG consists of two three-axis FGM mounted on a long boom, with one sensor (the inboard sensor) mounted about 6.3 m from the spacecraft bus and the other (the outboard sensor) mounted on the end of the 8.5 m boom. An improvement of the MAG capabilities compared to previous GOES Magnetometers was an increase in sampling rate to 10 samples/s. The reader is referred to Loto’aniu et al. (2019) for details of the MAG sensors, capabilities, on-orbit performance, and design improvements.
21.2 OBSERVING THE GEOMAGNETIC FIELD AT GEO The MAG science objectives for space weather operations include continuous observations of the geomagnetic field. Measuring the geomagnetic field at geosynchronous and geostationary orbits is important in both space weather operations and scientific research (Singer et al., 1996). This particular altitude enables the instrument to monitor major magnetospheric electrical current systems (shown in Fig. 21.1) that play significant roles in determining the level of geomagnetic activity (Ohtani et al., 2000). GOES Magnetometer data provides insight into space plasma physics processes that contribute to near-Earth space weather. Operationally, situational awareness provided by GOES magnetic observations often offers the first indication that significant space weather has reached or affected Earth’s magnetic field. The observations are also critical in the development of global models of the near-Earth space environment. Fig. 21.2 shows an example of the 1-min time series of the geomagnetic field viewed by NOAA Space Weather Prediction Center (SWPC) forecasters during space weather operations. On January 24, 2012, there was a dramatic sudden increase in the geomagnetic field values reaching over 150 nT observed by both GOES-13 and GOES-15 Magnetometers. This indicated the arrival at Earth of enhanced space weather due to material and electromagnetic energy emitted during a solar coronal mass ejection (CME).
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21. Magnetic Field Observations from the GOES-R Series
Solar wind-induced electric currents flowing in the magnetosphere
ic tail
Magnet
et
a she
Plasm
Ne
utr
Corotating plasma
al s
hee
t cu
rre
nt
Field-aligned current Ring current
Mag n curr etopa use ent
FIG. 21.1 Flow of major electrical current systems in Earth’s magnetosphere induced by solar wind-magnetospheric interactions. After Kivelson, M.G., Russell, C.T., 1996. Introduction to Space Physics, Cambridge University Press, USA.
GOES Magnetometer (1-min data)
Begin: 2012 Jan 22 0000 UTC
200
100
50
M 0 Jan 22
M
N
N Jan 23
M
M
N
Universal time Updated 2012 Jan 24 23:59:02 UTC
M
N
M
Jan 24
N
N
GOES 13 Hp Long W 75 GOES 15 Hp Long W 135
NanoTesla (nT)
150
Jan 25
NOAA/SWPC Boulder, CO USA
FIG. 21.2 Example GOES magnetic field observations viewed by the public NOAA SWPC webpage and SWPC forecasters of two geomagnetic storms. Courtesy of NOAA.
21.3 DATA PRODUCTS 21.3.1 Processing Levels There are three basic levels of data products all available in the Network Common Data Form-4 (netCDF4) format. The Level 0 (L0) data are unprocessed instrument telemetry at full resolution, the Level 1b (L1b) are full resolution instrument data with calibration corrections applied and converted into scientific units and the Level 2 or Level 2+ (L2+) products provide variables that have been derived to meet the needs of forecasters and the community. The raw
21.3 Data Products
253
MAG telemetry is transmitted from the spacecraft to the GOES-R ground system (GS) and calibrations are applied to the telemetry as part of the processing that converts the telemetry to usable data in scientific units (nano-Tesla).
21.3.2 One-Minute Averages The MAG space weather products include continuing the heritage 1-min geomagnetic field product (Singer et al., 1996) by averaging the 10-Hz MAG data. The averaging method is a simple boxcar method that gathers data over the specified time span and then divides the sum of the data by the total number of data points collected. A sliding boxcar average is a simple, well-understood method for calculating the average, and is the method used by previous GOES averages. For intervals where the measurements are uniformly distributed in time, this simple averaging technique provides an easily calculated and understood representation of the time series data. As with previous GOES averaging calculations, the earliest edge of the sliding window is used as the average value timestamp. The averaged values are based on the percentage of valid values in the averaging window. The minimum percent of valid values required for a valid average is usually set to 30%. If there are not enough minimum valid values to average then the average value is taken as the default error fill-in value (e.g., −9999).
21.3.3 Geomagnetic Field in Alternative Coordinates The full-resolution 10 Hz L1b MAG data files contain the magnetic field in up to five coordinate frames, listed in Table 21.1. The data are available in Earth Polar (Parallel) Normal (EPN) coordinates to comply with the coordinates used in legacy GOES magnetic field products (e.g., Singer et al., 1996). The Earth-Centered Inertial (ECI) coordinates are a commonly used and convenient celestial coordinate system that allows users to convert to other coordinate systems. The Body Reference Frame (BRF) coordinate system is produced to meet the requirements for magnetic field inputs for Space Environment In Situ Suite (SEISS) L2+ products (see Chapter 20). The data are also produced in the Attitude Control Reference Frame (ARCF) and Magnetometer Frame Inboard (MFIB)/Magnetometer Frame Outboard (MFOB) frames for postlaunch testing and calibration/validation studies. The individual MAG sensor observations are provided in all five frames, while composite values (i.e., either averaging the two sensors, combining the two sensors and subtracting the spacecraft dipole (i.e., gradiometric), or using only one of the two sensors) are provided in three coordinate frames (BRF, ECI, and EPN). In addition to these coordinate systems, at Level 2, we also provide MAG data in other geophysically significant coordinates. These coordinates, shown and described in Table 21.2, are local Magnetic Dipole Meridian (VDH), Geocentric Solar Ecliptic (GSE), and Geocentric Solar Magnetospheric (GSM). The coordinates and the algorithms needed to convert data into these frames can be found in the work of Russell (1971) and Hapgood (1992, 1995, 1997). At Level 2, the aforementioned composite values are in BRF, ECI, EPN, GSE, GSM, and VDH. The individual sensor data, in addition, is also provided in ACRF and sensor (MFIB/MFOB) coordinates.
TABLE 21.1 Full Resolution MAG L1b Coordinate Systems Abbreviation
Name
Description
EPN
Earth Polar (Parallel) Normal
P-axis is defined in the northward direction and normal to the orbit plane, E is the Earthwards (nadir) direction, and N completes the righthanded system (E x P). E and N lie on the orbit plane.
ECI
Earth-Centered Inertial
Defined relative to Earth Mean Equatorial with J2000 epoch (EME2000). The Z-axis points to Earth’s mean spin axis, X-axis points towards equinox of J2000 and the Y-axis completes the right-hand coordinate system.
BRF
Body Reference Frame
The local spacecraft attitude vector, defining spacecraft Yaw, Pitch, and Roll.
ACRF
Attitude Control Reference Frame
The coordinate frame that the spacecraft control system uses to point the instrument deck, the Earth Pointing Platform (EPP), hence the Earth Pointing Platform Reference Frame (EPRF), to nadir.
MFIB/MFOB
Magnetometer Frame Inboard/Magnetometer Calibrated orthogonal sensor frame. Frame Outboard
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TABLE 21.2 Alternative MAG L2+ Coordinate Systems Abbreviation
Name
GSE
Geocentric Solar Ecliptic
Description Z ecliptic pole
Earth
Y Dusk
X Sun
tic Eclip The X-axis is pointing from Earth towards the Sun. The X-axis and the Y-axis are included in the ecliptic plane. The Y-axis is pointing towards the dusk, opposing planetary motion. The Z-axis is parallel to the ecliptic pole. The GSE system has a yearly rotation with respect to the inertial system.
GSM
Z
Geocentric Solar Magnetospheric North magnetic pole
i Y Dusk Earth
X Sun The X-axis is pointing from Earth towards the Sun. The X-Z plane contains the dipole axis. The Y-axis is perpendicular to Earth’s magnetic dipole, towards the dusk and is included in the magnetic equatorial plane. The positive Z-axis is chosen to be in the same sense as the northern magnetic pole; the dipole tilt angle i is positive when the north magnetic pole is tilted towards the Sun. In addition to a yearly period due to the motion of Earth about the Sun, the GSM system rocks about the solar direction with a 24-h period. VDH
Z // North magnetic dipole axis
Magnetic Dipole Meridian
R
X
Point of observation
North magnetic pole
Outwards in dipole magnetic meridian
Y East
Earth
Z (H) is parallel to the north magnetic dipole axis. The X (V)-Z (H) plane contains the direction R of the point of observation, from Earth, and is a dipole magnetic meridian plane. The Y (D)-axis is perpendicular to the R vector, pointing eastwards. This system is a local coordinate system, which is dependent on the position of the point of observation relative to Earth.
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255
The need for use of multiple coordinate systems arises because different physical phenomena in near-Earth space can best be understood or described in different coordinate systems, and as previously noted, some coordinates are more useful for evaluating instrument performance. All the alternate coordinate systems are used extensively in the space physics and space weather communities. For example, the magnetic coordinates VDH and GSM are convenient for analyzing distortions of the magnetic field configuration such as field line stretching and dayside compression from the impinging solar wind. These distortions are often signatures of strong geomagnetic activity. GSE and GSM are very useful for studying solar wind-magnetospheric interactions and therefore in validating global models.
21.3.4 Quiet-Field Model When there is an active space weather period, the observed magnetic field deviates significantly from quiet-time magnetic field values. The quiet-time magnetic field values at the GOES-R orbit location can be estimated using a magnetic field model, and one of the operational L2 products that will be used in the GOES-R era is a continuous real-time comparison of the GOES-R MAG observations to a quiet-field model. Any significant difference between the modeled and observed values will suggest enhanced space weather activity. The quiet-field model is composed of an external component and an internal component. For the internal component, we use the International Geomagnetic Reference Field (IGRF) standard main field model adopted by the International Association of Geomagnetism and Aeronomy (IAGA). For more information on IGRF and IAGA, visit the IAGA Working Group V-MOD website at http://www.ngdc.noaa.gov/IAGA/vmod/. The external component is the OP-77 model, which is based on the work of Olson and Pfitzer (1974, 1977) and is valid for quiet-times and all tilt angles of the magnetic dipole of Earth. The quiet-field model values calculated at the GOES-R location is a simple addition of the IGRF model and contributions from external currents calculated using the OP-77 model. The model was implemented for GOES-R using the SpacePy python library developed by Los Alamos National Laboratory (LANL). The library can be found at https://spacepy.github.io/. An example of the quiet-field model product output is shown in Fig. 21.3. Deviations of the observed field from the quiet-field model indicates enhanced currents from different magnetospheric regions at different local times due to space weather activity.
21.3.5 Geosynchronous Magnetopause Crossing Detection The magnetopause of Earth is an abrupt boundary separating the geomagnetic field from the solar wind. This boundary is created by a balance between the solar wind pressure opposing the magnetic pressure of the geomagnetic field. The magnetopause distance from Earth continuously changes due to the dynamic nature of both the solar wind and Earth’s magnetic field.
FIG. 21.3 Comparison of GOES-16 magnetic field observations to OP77 model for January 11, 2017. The EPN coordinate frame is shown here and components are colored as E (red), P (green), N (blue), and Total (black). The OP77 model field is represented by ‘+’ symbols on the UT hour.
256
21. Magnetic Field Observations from the GOES-R Series
March 24, 1991 Day 83 300
GOES 6 Hp
200
nT
100 0
Magnetopause crossing
–100 –200 300
Shock
GOES 7 Hp
200
nT
100 0 –100 –200 00
02
04
06
08
10
12
14
16
18
20
22
UT (h) FIG. 21.4 GOES 6 and 7 magnetic field Hp component at the onset of the March 24, 1991 magnetic storm. After Singer, H.J., Matheson, L., Grubb, R., Newman, A., Bouwer, S.D., 1996. Monitoring space weather with the GOES Magnetometers. In: Washwell, E.R. (Ed.), GOES-8 and beyond, Proc. SPIE, vol. 2812, Int. Soc. for Opt. Eng., Bellingham, Wash., pp. 299–308.
For quiet space weather conditions, the magnetopause distance along the Earth-Sun line is usually 10–12 Earth radii (~63,710–76,452 km). However, when an enhanced space weather event affects Earth, the geomagnetic field becomes sufficiently compressed and eroded by the solar wind. If the event is strong enough it produces one of the most dramatic observations by the GOES Magnetometer where the solar facing outer edge of the magnetosphere (the magnetopause) compresses to below the orbit of GOES (6.6 Earth radii ~42,000 km). When this happens, there is a significant change in the GOES magnetic field observation. Fig. 21.4 shows an example of a magnetopause crossing observed in the Hp-component of the geomagnetic field by the GOES-6 and GOES-7 spacecraft during the onset of a very large geomagnetic storm on March 24, 1991. One of the L2 data products developed for the GOES-R era is an automatic geosynchronous magnetopause crossing (GMC) detection and location algorithm that will alert forecasters when this major event occurs. A GMC event is defined as when the magnetopause has been compressed and eroded to below geosynchronous orbit. The GMC detection algorithm is composed of three parts, with one using GOES Magnetometer data, the second using GOES particle data, and the third using solar wind data input into a magnetopause model. The need for three components is to provide better confidence that a GMC event has occurred and a short-time forecast using upstream solar wind monitor observations. GOES-R can only observe GMCs when it is located on the dayside, and the addition of a magnetopause model allows approximation of the magnetopause location when GOES-R is on the night side. The inclusion of GOES-R particle data increases our confidence that a GMC has occurred because the particle environment inside the magnetosphere is different from outside. The product outputs are state flags with values of either 1 or 0. These flags indicate the state of each part of the algorithm with 1 = magnetopause crossing and state 0 = no magnetopause crossing. Also included in the output are the calculated algorithm parameters and observations used to determine the magnetopause crossing state including magnetopause model outputs and particle ratios, magnetic field values, and solar wind parameters. The output displayed will be determined by NOAA/SWPC to best support the forecaster. An example of how the magnetic field portion of the product could be displayed is shown in Fig. 21.5 (Animation 21.1 of this figure is available in the online version at https://doi.org/10.1016/B978-0-12-814327-8.00021-4).
21.3 Data Products
257
FIG. 21.5 Example of a magnetopause detection algorithm output that could be displayed to forecasters. Animation 21.1 of this figure is available in the online version at https://doi.org/10.1016/B978-0-12-814327-8.00021-4.
21.3.5.1 Algorithm Overview As previously stated, the GOES-R GMC algorithm consists of three components. For the magnetic field part, a GMC occurs if H p < H p _ limit . where Hp is the GOES-R observed magnetic field P-component amplitude in EPN coordinates (see Table 21.1) and Hp_limit is the magnetic field P-component value below which GOES-R is observing a GMC. The value of Hp_limit is based on a high correlation between Hp and the interplanetary magnetic field-Bz component (IMF-Bz) during GMCs. It is assumed that GMCs occur only when IMF-Bz is negative; hence, Hp_limit is set to 0 nT. When a GMC occurs, the observed plasma ion and electron densities and temperatures change significantly. In particular, there is a substantial increase in the ratio of density to temperature. The GOES-R SEISS particle instrument suite L2+ products (see Chapter 20) include derived particle densities and temperatures. For the GMC algorithm, we define a GMC event as a substantial increase in these ratios for ions and electrons with energies 30 eV–30 keV. Suvorova et al. (2005) defined a GMC when the ion ratio is >30 and >100 for the electron ratio. They used LANL spacecraft particle data with slightly different energy ranges to GOES-R. Given the different energy ranges used and the fact that different instruments have different characteristics, for GOES-R we will conduct studies to determine the ratio values to define a GMC, but we expect the values to be close to those used by Suvorova et al. Three magnetopause models were tested and from these tests, we selected the Shue et al. (1998) model for use in the GMC algorithm. The model is a simple data-driven model taking as input the IMF-Bz component in nanoTeslas, the solar wind dynamic pressure in nanoPascals, and the spacecraft unit position vector from the center of Earth. As output, the model gives the magnetopause standoff distance (distance to magnetopause from Earth along the SunEarth line) and distance to the magnetopause from Earth through the spacecraft location. 21.3.5.2 Algorithm Outputs The outputs for the GMC algorithm are shown in Table 21.3. As previously stated, forecasters will determine the output display and Fig. 21.5 shows an example of how the magnetic field and magnetopause standoff distance portion of the product may be displayed. All the flags except for flag_r0 are only valid when GOES are on the dayside and this is a limitation of the product. If solar wind data are not available or invalid, the Shue magnetopause model output is invalid and therefore variables flag_r0, flag_r, shue_r, shue_r0, and shue_alpha are all invalid and the accuracy of the product is completely dependent on GOES observations and locations.
258
21. Magnetic Field Observations from the GOES-R Series
TABLE 21.3 The Magnetopause Detection Algorithm Output Name
Description
Data Type
flag_b_field
Set to 1 when magnetic field Hp-component is negative, else 0. Only valid when spacecraft is on dayside.
Integer
flag_ions
Set to 1 when ion density to temperature ratio is >30, else 0. Only valid when spacecraft is on dayside.
Integer
flag_electrons
Set to 1 when electron density to temperature ratio is >100, else 0. Only valid when spacecraft is on dayside.
Integer
flag_r
Set to 1 when magnetopause distance through the spacecraft location r is ≤ 6.6 Re. Calculated using the magnetopause model. Only valid when spacecraft is on dayside.
Integer
flag_r0
Set to 1 when magnetopause stand-off distance r0 is ≤ 6.6 Re. Calculated using the magnetopause model.
Integer
shue_r
Magnetopause distance through the spacecraft location r. Calculated using the magnetopause model.
Float
shue_r0
Magnetopause standoff distance r0. Calculated using the magnetopause model.
Float
shue_alpha
Magnetopause alpha (shape) parameter, Shue [1998].
Float
Hp
Magnetic field Hp-component.
Float
ratio_ions
Ratio of in situ ion density to ion temperature observed by spacecraft.
Float
ratio_electrons
Ratio of in situ electron density to electron temperature observed by spacecraft.
Float
Magnetic local time
GOES-R magnetic local time (MLT)
Time series (hh.mmm)
time
Universal Time of observation (UT)
Seconds since the J2K epoch
satellite
Satellite ID
Integer
21.4 CONCLUSIONS GOES-R Series MAG observations and space weather products represent an advancement compared to previous GOES geomagnetic field observational capabilities. MAG has a higher sample rate of 10 samples/s and the new MAG space weather products include providing data in different coordinate systems, calculating the model magnetic field at the spacecraft location and automatically detecting when GOES-R has crossed the magnetopause. Future MAG data improvements will include enhancements of these data products, development of new data products and further correction of the magnetic field data for known anomalies. GOES-R space weather product user guides and additional information are at the NOAA National Centers for Environmental Information website https://www.ngdc.noaa.gov/stp/satellite/goes-r.html. Additional documents and user resources can be found at the GOES-R Series website https://www.goes-r.gov/.
Acknowledgments We wish to acknowledge and thank the GOES-R Series Program Office. We also wish to thank the members of the NOAA NCEI GOES-R space weather group, which is part of the GOES-R Calibration Working Group (CWG) for their support to the NCEI MAG group. We also thank NOAA SWPC for additional MAG science subject matter expertise. The views, opinions, and findings contained in this report are those of the authors should not be construed as an official National Oceanic and Atmospheric Administration, National Aeronautics and Space Administration, or other US Government position, policy, or decision.
References Hapgood, M.A., 1992. Space physics coordinate transformations: a user guide. Planet. Space Sci. 40, 711–717. Hapgood, M.A., 1995. Space physics coordinate transformations: the role of precession. Ann. Geophys. 13, 713–716. Hapgood, M.A., 1997. Corrigendum to space physics coordinate transformations: a user guide. Planet. Space Sci. 45, 1047. Loto’aniu, T.M., Redmon, R.J., Califf, S., et al., 2019. The GOES-16 Spacecraft Science Magnetometer. Space Sci. Rev. 215, 32. https://doi. org/10.1007/s11214-019-0600-3.
FURTHER READING
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Ohtani, S.-I., Fujii, R., Hesse, M., Lysak, R.L., 2000. Magnetospheric current systems. Geophysical Monograph Series 118, https://doi.org/10.1029/ GM118. Olson, W.P., Pfitzer, K.A., 1974. A quantitative model of the Magnetospheric magnetic field. J. Geophys. Res. 79, 3739. Olson, W.P., Pfitzer, K.A., 1977. Magnetospheric magnetic field modeling. In: Annual Scientific Report, AFOSR Contract No. F44620-75-C-0033. Russell, C.T., 1971. Geophysical coordinate transformations. Cosmic. Electrodyn. 2, 184–196. Shue, J.‐H., et al., 1998. Magnetopause location under extreme solar wind conditions. J. Geophys. Res. 103 (A8), 17691–17700. https://doi. org/10.1029/98JA01103. Singer, H.J., Matheson, L., Grubb, R., Newman, A., Bouwer, S.D., 1996. Monitoring space weather with the GOES magnetometers, in GOES-8 and beyond. In: Washwell, E.R. (Ed.), Proc. SPIE. vol. 2812. Int. Soc. for Opt. Eng., Bellingham, Wash, pp. 299–308. Suvorova, A., Dmitriev, A., Chao, J.-K., Thomsen, M., Yang, Y.-H., 2005. Necessary conditions for geosynchronous magnetopause crossings. J. Geophys. Res. 110, A01206. https://doi.org/10.1029/2003JA010079.
Further Reading Kivelson, M.G., Russell, C.T., 1996. Introduction to Space Physics. Cambridge University Press, USA. Loto’aniu, T.M., Singer, H.J., 2009. GOES-R Magnetometer Convert Data to Alternate Geophysical Coordinate Systems Algorithm Theoretical Basis Document. (version 1.3).
21 Magnetic Field Observations from the GOES-R Series Paul T.M. Loto’aniu*,†, Samuel Califf*,†, Robert J. Redmon†, Howard J. Singer‡ *Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado-Boulder, Boulder, CO, United States, †NOAA National Centers for Environmental Information (NCEI), Boulder, CO, United States, ‡Space Weather Prediction Center, National Oceanic and Atmospheric Administration, Boulder, CO, United States
21.1 INTRODUCTION The Geostationary Operational Environmental Satellites (GOES)-R Series Magnetometer (MAG) continues the decades-long observations of Earth’s magnetic field at geostationary orbit by the National Oceanic and Atmospheric Administration (NOAA) GOES. These magnetic observations are among the most used space physics data sets in the world and are important for both space weather operations and for developing a better understanding of important space weather processes in the near-Earth space environment. The MAG consists of two fluxgate magnetometers (FGM) mounted on a boom. As with previous GOES, MAG consists of two three-axis FGM mounted on a long boom, with one sensor (the inboard sensor) mounted about 6.3 m from the spacecraft bus and the other (the outboard sensor) mounted on the end of the 8.5 m boom. An improvement of the MAG capabilities compared to previous GOES Magnetometers was an increase in sampling rate to 10 samples/s. The reader is referred to Loto’aniu et al. (2019) for details of the MAG sensors, capabilities, on-orbit performance, and design improvements.
21.2 OBSERVING THE GEOMAGNETIC FIELD AT GEO The MAG science objectives for space weather operations include continuous observations of the geomagnetic field. Measuring the geomagnetic field at geosynchronous and geostationary orbits is important in both space weather operations and scientific research (Singer et al., 1996). This particular altitude enables the instrument to monitor major magnetospheric electrical current systems (shown in Fig. 21.1) that play significant roles in determining the level of geomagnetic activity (Ohtani et al., 2000). GOES Magnetometer data provides insight into space plasma physics processes that contribute to near-Earth space weather. Operationally, situational awareness provided by GOES magnetic observations often offers the first indication that significant space weather has reached or affected Earth’s magnetic field. The observations are also critical in the development of global models of the near-Earth space environment. Fig. 21.2 shows an example of the 1-min time series of the geomagnetic field viewed by NOAA Space Weather Prediction Center (SWPC) forecasters during space weather operations. On January 24, 2012, there was a dramatic sudden increase in the geomagnetic field values reaching over 150 nT observed by both GOES-13 and GOES-15 Magnetometers. This indicated the arrival at Earth of enhanced space weather due to material and electromagnetic energy emitted during a solar coronal mass ejection (CME).
The GOES-R Series. https://doi.org/10.1016/B978-0-12-814327-8.00021-4
251
Copyright © 2020 Elsevier Inc. All rights reserved.
252
21. Magnetic Field Observations from the GOES-R Series
Solar wind-induced electric currents flowing in the magnetosphere
ic tail
Magnet
et
a she
Plasm
Ne
utr
Corotating plasma
al s
hee
t cu
rre
nt
Field-aligned current Ring current
Mag n curr etopa use ent
FIG. 21.1 Flow of major electrical current systems in Earth’s magnetosphere induced by solar wind-magnetospheric interactions. After Kivelson, M.G., Russell, C.T., 1996. Introduction to Space Physics, Cambridge University Press, USA.
GOES Magnetometer (1-min data)
Begin: 2012 Jan 22 0000 UTC
200
100
50
M 0 Jan 22
M
N
N Jan 23
M
M
N
Universal time Updated 2012 Jan 24 23:59:02 UTC
M
N
M
Jan 24
N
N
GOES 13 Hp Long W 75 GOES 15 Hp Long W 135
NanoTesla (nT)
150
Jan 25
NOAA/SWPC Boulder, CO USA
FIG. 21.2 Example GOES magnetic field observations viewed by the public NOAA SWPC webpage and SWPC forecasters of two geomagnetic storms. Courtesy of NOAA.
21.3 DATA PRODUCTS 21.3.1 Processing Levels There are three basic levels of data products all available in the Network Common Data Form-4 (netCDF4) format. The Level 0 (L0) data are unprocessed instrument telemetry at full resolution, the Level 1b (L1b) are full resolution instrument data with calibration corrections applied and converted into scientific units and the Level 2 or Level 2+ (L2+) products provide variables that have been derived to meet the needs of forecasters and the community. The raw
21.3 Data Products
253
MAG telemetry is transmitted from the spacecraft to the GOES-R ground system (GS) and calibrations are applied to the telemetry as part of the processing that converts the telemetry to usable data in scientific units (nano-Tesla).
21.3.2 One-Minute Averages The MAG space weather products include continuing the heritage 1-min geomagnetic field product (Singer et al., 1996) by averaging the 10-Hz MAG data. The averaging method is a simple boxcar method that gathers data over the specified time span and then divides the sum of the data by the total number of data points collected. A sliding boxcar average is a simple, well-understood method for calculating the average, and is the method used by previous GOES averages. For intervals where the measurements are uniformly distributed in time, this simple averaging technique provides an easily calculated and understood representation of the time series data. As with previous GOES averaging calculations, the earliest edge of the sliding window is used as the average value timestamp. The averaged values are based on the percentage of valid values in the averaging window. The minimum percent of valid values required for a valid average is usually set to 30%. If there are not enough minimum valid values to average then the average value is taken as the default error fill-in value (e.g., −9999).
21.3.3 Geomagnetic Field in Alternative Coordinates The full-resolution 10 Hz L1b MAG data files contain the magnetic field in up to five coordinate frames, listed in Table 21.1. The data are available in Earth Polar (Parallel) Normal (EPN) coordinates to comply with the coordinates used in legacy GOES magnetic field products (e.g., Singer et al., 1996). The Earth-Centered Inertial (ECI) coordinates are a commonly used and convenient celestial coordinate system that allows users to convert to other coordinate systems. The Body Reference Frame (BRF) coordinate system is produced to meet the requirements for magnetic field inputs for Space Environment In Situ Suite (SEISS) L2+ products (see Chapter 20). The data are also produced in the Attitude Control Reference Frame (ARCF) and Magnetometer Frame Inboard (MFIB)/Magnetometer Frame Outboard (MFOB) frames for postlaunch testing and calibration/validation studies. The individual MAG sensor observations are provided in all five frames, while composite values (i.e., either averaging the two sensors, combining the two sensors and subtracting the spacecraft dipole (i.e., gradiometric), or using only one of the two sensors) are provided in three coordinate frames (BRF, ECI, and EPN). In addition to these coordinate systems, at Level 2, we also provide MAG data in other geophysically significant coordinates. These coordinates, shown and described in Table 21.2, are local Magnetic Dipole Meridian (VDH), Geocentric Solar Ecliptic (GSE), and Geocentric Solar Magnetospheric (GSM). The coordinates and the algorithms needed to convert data into these frames can be found in the work of Russell (1971) and Hapgood (1992, 1995, 1997). At Level 2, the aforementioned composite values are in BRF, ECI, EPN, GSE, GSM, and VDH. The individual sensor data, in addition, is also provided in ACRF and sensor (MFIB/MFOB) coordinates.
TABLE 21.1 Full Resolution MAG L1b Coordinate Systems Abbreviation
Name
Description
EPN
Earth Polar (Parallel) Normal
P-axis is defined in the northward direction and normal to the orbit plane, E is the Earthwards (nadir) direction, and N completes the righthanded system (E x P). E and N lie on the orbit plane.
ECI
Earth-Centered Inertial
Defined relative to Earth Mean Equatorial with J2000 epoch (EME2000). The Z-axis points to Earth’s mean spin axis, X-axis points towards equinox of J2000 and the Y-axis completes the right-hand coordinate system.
BRF
Body Reference Frame
The local spacecraft attitude vector, defining spacecraft Yaw, Pitch, and Roll.
ACRF
Attitude Control Reference Frame
The coordinate frame that the spacecraft control system uses to point the instrument deck, the Earth Pointing Platform (EPP), hence the Earth Pointing Platform Reference Frame (EPRF), to nadir.
MFIB/MFOB
Magnetometer Frame Inboard/Magnetometer Calibrated orthogonal sensor frame. Frame Outboard
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TABLE 21.2 Alternative MAG L2+ Coordinate Systems Abbreviation
Name
GSE
Geocentric Solar Ecliptic
Description Z ecliptic pole
Earth
Y Dusk
X Sun
tic Eclip The X-axis is pointing from Earth towards the Sun. The X-axis and the Y-axis are included in the ecliptic plane. The Y-axis is pointing towards the dusk, opposing planetary motion. The Z-axis is parallel to the ecliptic pole. The GSE system has a yearly rotation with respect to the inertial system.
GSM
Z
Geocentric Solar Magnetospheric North magnetic pole
i Y Dusk Earth
X Sun The X-axis is pointing from Earth towards the Sun. The X-Z plane contains the dipole axis. The Y-axis is perpendicular to Earth’s magnetic dipole, towards the dusk and is included in the magnetic equatorial plane. The positive Z-axis is chosen to be in the same sense as the northern magnetic pole; the dipole tilt angle i is positive when the north magnetic pole is tilted towards the Sun. In addition to a yearly period due to the motion of Earth about the Sun, the GSM system rocks about the solar direction with a 24-h period. VDH
Z // North magnetic dipole axis
Magnetic Dipole Meridian
R
X
Point of observation
North magnetic pole
Outwards in dipole magnetic meridian
Y East
Earth
Z (H) is parallel to the north magnetic dipole axis. The X (V)-Z (H) plane contains the direction R of the point of observation, from Earth, and is a dipole magnetic meridian plane. The Y (D)-axis is perpendicular to the R vector, pointing eastwards. This system is a local coordinate system, which is dependent on the position of the point of observation relative to Earth.
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The need for use of multiple coordinate systems arises because different physical phenomena in near-Earth space can best be understood or described in different coordinate systems, and as previously noted, some coordinates are more useful for evaluating instrument performance. All the alternate coordinate systems are used extensively in the space physics and space weather communities. For example, the magnetic coordinates VDH and GSM are convenient for analyzing distortions of the magnetic field configuration such as field line stretching and dayside compression from the impinging solar wind. These distortions are often signatures of strong geomagnetic activity. GSE and GSM are very useful for studying solar wind-magnetospheric interactions and therefore in validating global models.
21.3.4 Quiet-Field Model When there is an active space weather period, the observed magnetic field deviates significantly from quiet-time magnetic field values. The quiet-time magnetic field values at the GOES-R orbit location can be estimated using a magnetic field model, and one of the operational L2 products that will be used in the GOES-R era is a continuous real-time comparison of the GOES-R MAG observations to a quiet-field model. Any significant difference between the modeled and observed values will suggest enhanced space weather activity. The quiet-field model is composed of an external component and an internal component. For the internal component, we use the International Geomagnetic Reference Field (IGRF) standard main field model adopted by the International Association of Geomagnetism and Aeronomy (IAGA). For more information on IGRF and IAGA, visit the IAGA Working Group V-MOD website at http://www.ngdc.noaa.gov/IAGA/vmod/. The external component is the OP-77 model, which is based on the work of Olson and Pfitzer (1974, 1977) and is valid for quiet-times and all tilt angles of the magnetic dipole of Earth. The quiet-field model values calculated at the GOES-R location is a simple addition of the IGRF model and contributions from external currents calculated using the OP-77 model. The model was implemented for GOES-R using the SpacePy python library developed by Los Alamos National Laboratory (LANL). The library can be found at https://spacepy.github.io/. An example of the quiet-field model product output is shown in Fig. 21.3. Deviations of the observed field from the quiet-field model indicates enhanced currents from different magnetospheric regions at different local times due to space weather activity.
21.3.5 Geosynchronous Magnetopause Crossing Detection The magnetopause of Earth is an abrupt boundary separating the geomagnetic field from the solar wind. This boundary is created by a balance between the solar wind pressure opposing the magnetic pressure of the geomagnetic field. The magnetopause distance from Earth continuously changes due to the dynamic nature of both the solar wind and Earth’s magnetic field.
FIG. 21.3 Comparison of GOES-16 magnetic field observations to OP77 model for January 11, 2017. The EPN coordinate frame is shown here and components are colored as E (red), P (green), N (blue), and Total (black). The OP77 model field is represented by ‘+’ symbols on the UT hour.
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21. Magnetic Field Observations from the GOES-R Series
March 24, 1991 Day 83 300
GOES 6 Hp
200
nT
100 0
Magnetopause crossing
–100 –200 300
Shock
GOES 7 Hp
200
nT
100 0 –100 –200 00
02
04
06
08
10
12
14
16
18
20
22
UT (h) FIG. 21.4 GOES 6 and 7 magnetic field Hp component at the onset of the March 24, 1991 magnetic storm. After Singer, H.J., Matheson, L., Grubb, R., Newman, A., Bouwer, S.D., 1996. Monitoring space weather with the GOES Magnetometers. In: Washwell, E.R. (Ed.), GOES-8 and beyond, Proc. SPIE, vol. 2812, Int. Soc. for Opt. Eng., Bellingham, Wash., pp. 299–308.
For quiet space weather conditions, the magnetopause distance along the Earth-Sun line is usually 10–12 Earth radii (~63,710–76,452 km). However, when an enhanced space weather event affects Earth, the geomagnetic field becomes sufficiently compressed and eroded by the solar wind. If the event is strong enough it produces one of the most dramatic observations by the GOES Magnetometer where the solar facing outer edge of the magnetosphere (the magnetopause) compresses to below the orbit of GOES (6.6 Earth radii ~42,000 km). When this happens, there is a significant change in the GOES magnetic field observation. Fig. 21.4 shows an example of a magnetopause crossing observed in the Hp-component of the geomagnetic field by the GOES-6 and GOES-7 spacecraft during the onset of a very large geomagnetic storm on March 24, 1991. One of the L2 data products developed for the GOES-R era is an automatic geosynchronous magnetopause crossing (GMC) detection and location algorithm that will alert forecasters when this major event occurs. A GMC event is defined as when the magnetopause has been compressed and eroded to below geosynchronous orbit. The GMC detection algorithm is composed of three parts, with one using GOES Magnetometer data, the second using GOES particle data, and the third using solar wind data input into a magnetopause model. The need for three components is to provide better confidence that a GMC event has occurred and a short-time forecast using upstream solar wind monitor observations. GOES-R can only observe GMCs when it is located on the dayside, and the addition of a magnetopause model allows approximation of the magnetopause location when GOES-R is on the night side. The inclusion of GOES-R particle data increases our confidence that a GMC has occurred because the particle environment inside the magnetosphere is different from outside. The product outputs are state flags with values of either 1 or 0. These flags indicate the state of each part of the algorithm with 1 = magnetopause crossing and state 0 = no magnetopause crossing. Also included in the output are the calculated algorithm parameters and observations used to determine the magnetopause crossing state including magnetopause model outputs and particle ratios, magnetic field values, and solar wind parameters. The output displayed will be determined by NOAA/SWPC to best support the forecaster. An example of how the magnetic field portion of the product could be displayed is shown in Fig. 21.5 (Animation 21.1 of this figure is available in the online version at https://doi.org/10.1016/B978-0-12-814327-8.00021-4).
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FIG. 21.5 Example of a magnetopause detection algorithm output that could be displayed to forecasters. Animation 21.1 of this figure is available in the online version at https://doi.org/10.1016/B978-0-12-814327-8.00021-4.
21.3.5.1 Algorithm Overview As previously stated, the GOES-R GMC algorithm consists of three components. For the magnetic field part, a GMC occurs if H p < H p _ limit . where Hp is the GOES-R observed magnetic field P-component amplitude in EPN coordinates (see Table 21.1) and Hp_limit is the magnetic field P-component value below which GOES-R is observing a GMC. The value of Hp_limit is based on a high correlation between Hp and the interplanetary magnetic field-Bz component (IMF-Bz) during GMCs. It is assumed that GMCs occur only when IMF-Bz is negative; hence, Hp_limit is set to 0 nT. When a GMC occurs, the observed plasma ion and electron densities and temperatures change significantly. In particular, there is a substantial increase in the ratio of density to temperature. The GOES-R SEISS particle instrument suite L2+ products (see Chapter 20) include derived particle densities and temperatures. For the GMC algorithm, we define a GMC event as a substantial increase in these ratios for ions and electrons with energies 30 eV–30 keV. Suvorova et al. (2005) defined a GMC when the ion ratio is >30 and >100 for the electron ratio. They used LANL spacecraft particle data with slightly different energy ranges to GOES-R. Given the different energy ranges used and the fact that different instruments have different characteristics, for GOES-R we will conduct studies to determine the ratio values to define a GMC, but we expect the values to be close to those used by Suvorova et al. Three magnetopause models were tested and from these tests, we selected the Shue et al. (1998) model for use in the GMC algorithm. The model is a simple data-driven model taking as input the IMF-Bz component in nanoTeslas, the solar wind dynamic pressure in nanoPascals, and the spacecraft unit position vector from the center of Earth. As output, the model gives the magnetopause standoff distance (distance to magnetopause from Earth along the SunEarth line) and distance to the magnetopause from Earth through the spacecraft location. 21.3.5.2 Algorithm Outputs The outputs for the GMC algorithm are shown in Table 21.3. As previously stated, forecasters will determine the output display and Fig. 21.5 shows an example of how the magnetic field and magnetopause standoff distance portion of the product may be displayed. All the flags except for flag_r0 are only valid when GOES are on the dayside and this is a limitation of the product. If solar wind data are not available or invalid, the Shue magnetopause model output is invalid and therefore variables flag_r0, flag_r, shue_r, shue_r0, and shue_alpha are all invalid and the accuracy of the product is completely dependent on GOES observations and locations.
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21. Magnetic Field Observations from the GOES-R Series
TABLE 21.3 The Magnetopause Detection Algorithm Output Name
Description
Data Type
flag_b_field
Set to 1 when magnetic field Hp-component is negative, else 0. Only valid when spacecraft is on dayside.
Integer
flag_ions
Set to 1 when ion density to temperature ratio is >30, else 0. Only valid when spacecraft is on dayside.
Integer
flag_electrons
Set to 1 when electron density to temperature ratio is >100, else 0. Only valid when spacecraft is on dayside.
Integer
flag_r
Set to 1 when magnetopause distance through the spacecraft location r is ≤ 6.6 Re. Calculated using the magnetopause model. Only valid when spacecraft is on dayside.
Integer
flag_r0
Set to 1 when magnetopause stand-off distance r0 is ≤ 6.6 Re. Calculated using the magnetopause model.
Integer
shue_r
Magnetopause distance through the spacecraft location r. Calculated using the magnetopause model.
Float
shue_r0
Magnetopause standoff distance r0. Calculated using the magnetopause model.
Float
shue_alpha
Magnetopause alpha (shape) parameter, Shue [1998].
Float
Hp
Magnetic field Hp-component.
Float
ratio_ions
Ratio of in situ ion density to ion temperature observed by spacecraft.
Float
ratio_electrons
Ratio of in situ electron density to electron temperature observed by spacecraft.
Float
Magnetic local time
GOES-R magnetic local time (MLT)
Time series (hh.mmm)
time
Universal Time of observation (UT)
Seconds since the J2K epoch
satellite
Satellite ID
Integer
21.4 CONCLUSIONS GOES-R Series MAG observations and space weather products represent an advancement compared to previous GOES geomagnetic field observational capabilities. MAG has a higher sample rate of 10 samples/s and the new MAG space weather products include providing data in different coordinate systems, calculating the model magnetic field at the spacecraft location and automatically detecting when GOES-R has crossed the magnetopause. Future MAG data improvements will include enhancements of these data products, development of new data products and further correction of the magnetic field data for known anomalies. GOES-R space weather product user guides and additional information are at the NOAA National Centers for Environmental Information website https://www.ngdc.noaa.gov/stp/satellite/goes-r.html. Additional documents and user resources can be found at the GOES-R Series website https://www.goes-r.gov/.
Acknowledgments We wish to acknowledge and thank the GOES-R Series Program Office. We also wish to thank the members of the NOAA NCEI GOES-R space weather group, which is part of the GOES-R Calibration Working Group (CWG) for their support to the NCEI MAG group. We also thank NOAA SWPC for additional MAG science subject matter expertise. The views, opinions, and findings contained in this report are those of the authors should not be construed as an official National Oceanic and Atmospheric Administration, National Aeronautics and Space Administration, or other US Government position, policy, or decision.
References Hapgood, M.A., 1992. Space physics coordinate transformations: a user guide. Planet. Space Sci. 40, 711–717. Hapgood, M.A., 1995. Space physics coordinate transformations: the role of precession. Ann. Geophys. 13, 713–716. Hapgood, M.A., 1997. Corrigendum to space physics coordinate transformations: a user guide. Planet. Space Sci. 45, 1047. Loto’aniu, T.M., Redmon, R.J., Califf, S., et al., 2019. The GOES-16 Spacecraft Science Magnetometer. Space Sci. Rev. 215, 32. https://doi. org/10.1007/s11214-019-0600-3.
FURTHER READING
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Ohtani, S.-I., Fujii, R., Hesse, M., Lysak, R.L., 2000. Magnetospheric current systems. Geophysical Monograph Series 118, https://doi.org/10.1029/ GM118. Olson, W.P., Pfitzer, K.A., 1974. A quantitative model of the Magnetospheric magnetic field. J. Geophys. Res. 79, 3739. Olson, W.P., Pfitzer, K.A., 1977. Magnetospheric magnetic field modeling. In: Annual Scientific Report, AFOSR Contract No. F44620-75-C-0033. Russell, C.T., 1971. Geophysical coordinate transformations. Cosmic. Electrodyn. 2, 184–196. Shue, J.‐H., et al., 1998. Magnetopause location under extreme solar wind conditions. J. Geophys. Res. 103 (A8), 17691–17700. https://doi. org/10.1029/98JA01103. Singer, H.J., Matheson, L., Grubb, R., Newman, A., Bouwer, S.D., 1996. Monitoring space weather with the GOES magnetometers, in GOES-8 and beyond. In: Washwell, E.R. (Ed.), Proc. SPIE. vol. 2812. Int. Soc. for Opt. Eng., Bellingham, Wash, pp. 299–308. Suvorova, A., Dmitriev, A., Chao, J.-K., Thomsen, M., Yang, Y.-H., 2005. Necessary conditions for geosynchronous magnetopause crossings. J. Geophys. Res. 110, A01206. https://doi.org/10.1029/2003JA010079.
Further Reading Kivelson, M.G., Russell, C.T., 1996. Introduction to Space Physics. Cambridge University Press, USA. Loto’aniu, T.M., Singer, H.J., 2009. GOES-R Magnetometer Convert Data to Alternate Geophysical Coordinate Systems Algorithm Theoretical Basis Document. (version 1.3).