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Solar-proton polar-cap intensity structures as a test of magnetic field models
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Adv. Space Rex Vol. 28, No. 12, pp. 17.53-1757,200l 0 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-I 177/O! $20.00 + 0.00 PII: SO273-1177(01)00542-7
SOLAR-PROTON POLAR-CAP INTENSITY STRUCTURES AS A TEST OF MAGNETIC FIELD MODELS J. B. Blake, M. C. McNab, and J. E. Mazur The Aerospace Corporation Los Angeles, CA 90009, USA
ABSTRACT The entry of energetic solar protons to the polar caps offers an interesting way to test models of the geomagnetic field. In this brief report, we present a comparison between SAMPEX observations of solar-particle intensity structure during a polar cap traversal with numerical trajectory calculations using the IGBF + T96 field model. 0 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved. INTRODUCTION Many studies were carried out in the 1960s and early 1970s of the access of solar particles to the polar regions of the Earth. These studies were motivated by the complex structure often seen in the solar-particle fluxes as a spacecraft crossed a polar cap. The structures seen included: intensity variations, often large, in the polar cap as a function of latitude and local time; pronounced North-South differences in particle intensities and latitudinal features; and variations in the geomagnetic cutoff as a function of local time and geomagnetic activity. Marked differences were seen, especially early in a solar particle event, which frequently are time periods of large interplanetary particle anisotropies. A comprehensive review of this early work was given by Paul&as (1974). These polar-cap observations were interpreted in terms of direct entry by motion along trajectories without scattering, diffusive entry, or a combination of both. The configuration of the geomagnetic field, of course, plays a crucial part in determining polar-cap intensity structure and geomagnetic cutoff location. In this paper, we take a first look at comparisons between the proton intensity structure as seen by SAMPEX during the SEP event of early November 1997 with numerical trajectory calculations using the IGRF + T96 model (NSSDC) of the Earth’s magnetic field. DATA AND CALCULATIONS Figure 1 shows an example of such a calculation. In this case the proton enters near the nose of the magnetosphere and executes a complex trajectory before reaching SAMPEX at low altitude over the northern polar cap (of course the actual calculation shown traced the proton backward from SAMPEX to the magnetopause.) Figure 2 shows the countrate of energetic protons in the energy range from g-18 MeV over the polar caps for the first five SAMPEX orbits of Day 311 of 1997 as measured by the MAST instrument (Cook et al., 1993). Marked latitudinal structure was observed throughout this time period, and the southern polar cap maintained a large central depression for several hours. Figure 3 shows a single southern polar-cap traverse from Figure 2. The proton countrate every six seconds is plotted, and the counting statistics are shown. It can be seen that statistical errors are negligible. We study this particular polar-cap traversal in this report. We calculated the trajectory of a 13-MeV proton back in time every six seconds along the orbital path of SAMPEX using the s/c latitude, longitude, altitude, and viewing direction. The IGRF + T96 model was used with the measured, time-varying values of Dst, solar-wind speed and density, and interplanetary magnetic-field components as field model input. Gn the order of 200 such tracings were made for this single polar-cap traverse. Figure 4 shows the polar-cap data of Figure 3 overplotted with the Xgsm, Ygsm, and Zgsm of the calculated point where the solar proton either crossed the magnetopause or came sunward up the magnetotail on its path to SAMPEX. Some interesting transitions are labeled with letters at the top of the plot. The two triangles, at A and
1753
J. B. Blake et al.
Noon-Midnight Meridional Plane
Equatorial Plane lo-
52 B > I -5 -
-10 -
Fig. 1. A sample trajectory from SAhPEX to magnetopause is shown.
1755
Solar Protons as Magnetic Field Model Test
ki M
‘0
2000 N S
UT, see Fig. 2. The countrate of energetic solar protons (g-18 MeV) is plotted for the first ten polar-cap crossings of SAMPEX on Day 311 of 1997. Data are plotted only when SAMPEX was at invariant latitudes above 55’. A highly complex, time dependent structure can be seen in the proton countrates as the polar cap was traversed. 85
,...,“‘,“‘,...,“‘,.
.
/ / / / / / / / f
:
9200
9400
9600
9800
10000 10200
UT, set Fig. 3. A single polar-cap traversal is shown on an expanded scale; a point is plotted every six seconds. The counting statistics are shown, demonstrating that the complex structure is not statistical in origin.
1756
J. B. Blakeet al.
A
B
C
D
EFG 20 10 OR
E
-10 -20 -30 -40 -50
9ooo
9200
-60
9400 9600 9800 loo00 1o2oo UT of Day 97311
Fig. 4. The time history profile of Figure 3 is overplotted with the calculated entry point of the solar protons at each point in the countrate curve. The two triangles indicate the location of the calculated geomagnetic cutoff. The labels are referenced in the text. G, are the calculated positions of the geomagnetic cutoff on either side of the polar cap. It can be seen that the actual cutoffs are a bit lower in latitude than the calculated one. The largest, most abrupt transition occurs at B, where the intensity decreases by a factor of more than 5. At that point, the access point is moving rapidly down the tail; early work often attributed changes in particle intensity to a transition between access at the flanks of the magnetosphere and from the deep tail. At point C, there is an abrupt rise of a factor of about 3; it is associated with a change in Y from dusk to dawn and a change in Z from the equator southward. During this transition, X remains deep down the magnetotail. At D, the solar particle intensity returns to the pre-C value. At this point, X moves abruptly earthward, and Z starts to move equatorward. The particle intensity starts to increase slowly after D toward the level prior to B. This increase is interupted at E by a drop associated with an abrupt transition in Y from dusk to dawn. The solar particle intensity then increases again and reaches at F the level prior to B. Figure 5 presents the calculated angle between the proton velocity vector and the magnetopause surface from Schulz and McNab (1996). The peak in the center of the polar cap around 9600 s (between C and D in Figure 4) occurs during the time period where the penetration angle is small. A small penetration angle around 9900 s is associated with a decrease in the proton intensity, not an increase. One might argue that a solar proton gyrating around an interplanetary field with small pitch angle could find itself inside the magnetopause moving along and about a roughly parallel geomagnetic field line. If the geomagnetic field were significantly larger in magnitude, the gyrocenter of the solar particle would move inward. The results given above were for a proton energy of 13 MeV; we checked the energy (rigidity) dependence of the results by making identical calculations for the minimum and maximum proton energies of the SAMPEX detector; 8 and 18 MeV. There was a negligible difference in the location of the access points, indicating that all protons within the 8-18 MeV energy channel entered the magnetosphere at the same location.
DISCUSSION We are encouraged by these results: abrupt changes in particle intensity can be associated fairly closely with marked changes in the location of solar particle access to the magnetosphere as computed using a geomagnetic field model widely employed in space plasma physics, derived from data and used in ways remote from solar energetic particle access to the magnetosphere.
Solar Protons as Magnetic Field Model Test
A
B
C
1151
D
?
G
-
CUtOff
9000
!mo
9400
9600
9800
MagnetopausaPentration
loo00
Angle
10200
UT, set Fig. 5. Calculated angle of incidence of solar protons upon the magnetopause that reach SAMPEX is plotted at six second intervals over the same polar-cap traversal as Figure 4.
These studies will be expanded in two ways. First, we will perform similar calculations for all the polar-cap traversals in this event and will use different geomagnetic field models. The goals, of course, are to determine whether there is some consistent correlation between the intensity structure and the calculated access points, and to determine whether there are significant differences between the field models in common use when used to calculate solar particle access. The pitch-angle distribution of the solar energetic particles and the direction of the interplanetary magnetic field also are parameters of obvious interest; these correlations will be explored as well. ACKNOWLEDGEMENTS This work was supported at The Aerospace Corporation by NASA under NASA Cooperative Agreement 26979B.
Cook, W. R., A. C. Cummings, J. R. Cummings, T. L. Gerrard, B. Kecman, R. A. Mewaldt, R. S. Se.esnick, E. C. Stone, and T. T. von Rosenvinge, MAST:A Mass Spectrometer Telescope for Studies of the Isotopic Composition of Solar, Anomalous, and Galactic Cosmic Ray Nuclei, IEEE Trans. On geoscience and Remote Sensing, 31,557, 1993. NSSDC - software code that implements the Tsyganenko T96 external geomagnetic field model was obtained from the National Space Science Data Center at the NASA Goddard Space Flight Center. Paulikas, G. A., Tracing of High-Latitude Magnetic Field Lines by Solar Particles, Reviews of Geophysics and Space Physics, 12, 117-128,1974. Schulz, M. and M. C. McNab, Source-Surface Modeling of Planetary Magnetospheres, Journal Geophys. Res. 101, 50955118,19%.
Adv. Space Rex Vol. 28, No. 12, pp. 17.53-1757,200l 0 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-I 177/O! $20.00 + 0.00 PII: SO273-1177(01)00542-7
SOLAR-PROTON POLAR-CAP INTENSITY STRUCTURES AS A TEST OF MAGNETIC FIELD MODELS J. B. Blake, M. C. McNab, and J. E. Mazur The Aerospace Corporation Los Angeles, CA 90009, USA
ABSTRACT The entry of energetic solar protons to the polar caps offers an interesting way to test models of the geomagnetic field. In this brief report, we present a comparison between SAMPEX observations of solar-particle intensity structure during a polar cap traversal with numerical trajectory calculations using the IGBF + T96 field model. 0 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved. INTRODUCTION Many studies were carried out in the 1960s and early 1970s of the access of solar particles to the polar regions of the Earth. These studies were motivated by the complex structure often seen in the solar-particle fluxes as a spacecraft crossed a polar cap. The structures seen included: intensity variations, often large, in the polar cap as a function of latitude and local time; pronounced North-South differences in particle intensities and latitudinal features; and variations in the geomagnetic cutoff as a function of local time and geomagnetic activity. Marked differences were seen, especially early in a solar particle event, which frequently are time periods of large interplanetary particle anisotropies. A comprehensive review of this early work was given by Paul&as (1974). These polar-cap observations were interpreted in terms of direct entry by motion along trajectories without scattering, diffusive entry, or a combination of both. The configuration of the geomagnetic field, of course, plays a crucial part in determining polar-cap intensity structure and geomagnetic cutoff location. In this paper, we take a first look at comparisons between the proton intensity structure as seen by SAMPEX during the SEP event of early November 1997 with numerical trajectory calculations using the IGRF + T96 model (NSSDC) of the Earth’s magnetic field. DATA AND CALCULATIONS Figure 1 shows an example of such a calculation. In this case the proton enters near the nose of the magnetosphere and executes a complex trajectory before reaching SAMPEX at low altitude over the northern polar cap (of course the actual calculation shown traced the proton backward from SAMPEX to the magnetopause.) Figure 2 shows the countrate of energetic protons in the energy range from g-18 MeV over the polar caps for the first five SAMPEX orbits of Day 311 of 1997 as measured by the MAST instrument (Cook et al., 1993). Marked latitudinal structure was observed throughout this time period, and the southern polar cap maintained a large central depression for several hours. Figure 3 shows a single southern polar-cap traverse from Figure 2. The proton countrate every six seconds is plotted, and the counting statistics are shown. It can be seen that statistical errors are negligible. We study this particular polar-cap traversal in this report. We calculated the trajectory of a 13-MeV proton back in time every six seconds along the orbital path of SAMPEX using the s/c latitude, longitude, altitude, and viewing direction. The IGRF + T96 model was used with the measured, time-varying values of Dst, solar-wind speed and density, and interplanetary magnetic-field components as field model input. Gn the order of 200 such tracings were made for this single polar-cap traverse. Figure 4 shows the polar-cap data of Figure 3 overplotted with the Xgsm, Ygsm, and Zgsm of the calculated point where the solar proton either crossed the magnetopause or came sunward up the magnetotail on its path to SAMPEX. Some interesting transitions are labeled with letters at the top of the plot. The two triangles, at A and
1753
J. B. Blake et al.
Noon-Midnight Meridional Plane
Equatorial Plane lo-
52 B > I -5 -
-10 -
Fig. 1. A sample trajectory from SAhPEX to magnetopause is shown.
1755
Solar Protons as Magnetic Field Model Test
ki M
‘0
2000 N S
UT, see Fig. 2. The countrate of energetic solar protons (g-18 MeV) is plotted for the first ten polar-cap crossings of SAMPEX on Day 311 of 1997. Data are plotted only when SAMPEX was at invariant latitudes above 55’. A highly complex, time dependent structure can be seen in the proton countrates as the polar cap was traversed. 85
,...,“‘,“‘,...,“‘,.
.
/ / / / / / / / f
:
9200
9400
9600
9800
10000 10200
UT, set Fig. 3. A single polar-cap traversal is shown on an expanded scale; a point is plotted every six seconds. The counting statistics are shown, demonstrating that the complex structure is not statistical in origin.
1756
J. B. Blakeet al.
A
B
C
D
EFG 20 10 OR
E
-10 -20 -30 -40 -50
9ooo
9200
-60
9400 9600 9800 loo00 1o2oo UT of Day 97311
Fig. 4. The time history profile of Figure 3 is overplotted with the calculated entry point of the solar protons at each point in the countrate curve. The two triangles indicate the location of the calculated geomagnetic cutoff. The labels are referenced in the text. G, are the calculated positions of the geomagnetic cutoff on either side of the polar cap. It can be seen that the actual cutoffs are a bit lower in latitude than the calculated one. The largest, most abrupt transition occurs at B, where the intensity decreases by a factor of more than 5. At that point, the access point is moving rapidly down the tail; early work often attributed changes in particle intensity to a transition between access at the flanks of the magnetosphere and from the deep tail. At point C, there is an abrupt rise of a factor of about 3; it is associated with a change in Y from dusk to dawn and a change in Z from the equator southward. During this transition, X remains deep down the magnetotail. At D, the solar particle intensity returns to the pre-C value. At this point, X moves abruptly earthward, and Z starts to move equatorward. The particle intensity starts to increase slowly after D toward the level prior to B. This increase is interupted at E by a drop associated with an abrupt transition in Y from dusk to dawn. The solar particle intensity then increases again and reaches at F the level prior to B. Figure 5 presents the calculated angle between the proton velocity vector and the magnetopause surface from Schulz and McNab (1996). The peak in the center of the polar cap around 9600 s (between C and D in Figure 4) occurs during the time period where the penetration angle is small. A small penetration angle around 9900 s is associated with a decrease in the proton intensity, not an increase. One might argue that a solar proton gyrating around an interplanetary field with small pitch angle could find itself inside the magnetopause moving along and about a roughly parallel geomagnetic field line. If the geomagnetic field were significantly larger in magnitude, the gyrocenter of the solar particle would move inward. The results given above were for a proton energy of 13 MeV; we checked the energy (rigidity) dependence of the results by making identical calculations for the minimum and maximum proton energies of the SAMPEX detector; 8 and 18 MeV. There was a negligible difference in the location of the access points, indicating that all protons within the 8-18 MeV energy channel entered the magnetosphere at the same location.
DISCUSSION We are encouraged by these results: abrupt changes in particle intensity can be associated fairly closely with marked changes in the location of solar particle access to the magnetosphere as computed using a geomagnetic field model widely employed in space plasma physics, derived from data and used in ways remote from solar energetic particle access to the magnetosphere.
Solar Protons as Magnetic Field Model Test
A
B
C
1151
D
?
G
-
CUtOff
9000
!mo
9400
9600
9800
MagnetopausaPentration
loo00
Angle
10200
UT, set Fig. 5. Calculated angle of incidence of solar protons upon the magnetopause that reach SAMPEX is plotted at six second intervals over the same polar-cap traversal as Figure 4.
These studies will be expanded in two ways. First, we will perform similar calculations for all the polar-cap traversals in this event and will use different geomagnetic field models. The goals, of course, are to determine whether there is some consistent correlation between the intensity structure and the calculated access points, and to determine whether there are significant differences between the field models in common use when used to calculate solar particle access. The pitch-angle distribution of the solar energetic particles and the direction of the interplanetary magnetic field also are parameters of obvious interest; these correlations will be explored as well. ACKNOWLEDGEMENTS This work was supported at The Aerospace Corporation by NASA under NASA Cooperative Agreement 26979B.
Cook, W. R., A. C. Cummings, J. R. Cummings, T. L. Gerrard, B. Kecman, R. A. Mewaldt, R. S. Se.esnick, E. C. Stone, and T. T. von Rosenvinge, MAST:A Mass Spectrometer Telescope for Studies of the Isotopic Composition of Solar, Anomalous, and Galactic Cosmic Ray Nuclei, IEEE Trans. On geoscience and Remote Sensing, 31,557, 1993. NSSDC - software code that implements the Tsyganenko T96 external geomagnetic field model was obtained from the National Space Science Data Center at the NASA Goddard Space Flight Center. Paulikas, G. A., Tracing of High-Latitude Magnetic Field Lines by Solar Particles, Reviews of Geophysics and Space Physics, 12, 117-128,1974. Schulz, M. and M. C. McNab, Source-Surface Modeling of Planetary Magnetospheres, Journal Geophys. Res. 101, 50955118,19%.