- Email: [email protected]
Monitoring the heliospheric current sheet local structure for the years 1995 to 2001
ARTICLE IN PRESS
Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 226–233 www.elsevier.com/locate/jastp
Monitoring the heliospheric current sheet local structure for the years 1995 to 2001 J.J. Blanco, J. Rodriguez-Pacheco, M.A. Hidalgo, J. Sequeiros Space Research Group, Dpto. Fisica, Univ. Alcala, Alcala de Henares-Madrid, Spain Accepted 27 August 2007 Available online 1 October 2007
Abstract We have monitored the heliospheric current sheet (HCS) local structure through a 6.5 year period starting in January 1995. This interval begins with near solar minimum conditions and finishes in solar maximum conditions. We have used data from Wind mission, mainly the magnetic field instrument (MFI) and the solar wind experiment (SWE). Our work is focused on the HCS local inclination and the solar wind conditions around the HCS crossings, with a particular interest on their evolution along the ascending phase of solar cycle 23 and its relationship with solar wind phenomena such as magnetic cloud and stream interaction regions. We defined a real HCS crossing when a magnetic field minimum, showing a polarity reversal is observed and QeB (where Qe is the electron heat flux in solar wind) reverses its sign through an interval no longer than 60 min. The results suggest that the HCS local structure is more dependent on the solar wind conditions than on the solar cycle stage. r 2007 Elsevier Ltd. All rights reserved. Keywords: Heliospheric current sheet; Solar cycle; Stream interaction region; Magnetic cloud; Slow solar wind
1. Introduction The heliospheric current sheet (HCS) is probably the most relevant feature immersed the solar wind. It is present along all the solar cycle although its global shape suffers important changes, particularly close to the solar maximum (Smith, 2001; Riley et al., 2002). The HCS goes into the heliosphere from the cusp of the magnetic arcs that form the helmet streamer belt. So any change in the helmet streamer should affect to the global shape of the HCS, this is clear during the solar maximum when Corresponding author. Tel.: +34 91 8855053; fax: +34 91 8854942. E-mail address: [email protected] (J.J. Blanco).
1364-6826/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2007.08.030
the helmet streamer fills wide corona regions (Bavassano et al., 1997; Wang et al., 2000). At 1 AU the HCS is observed as a sharp boundary between regions with opposite magnetic polarity dividing the heliosphere in magnetic sectors. These sectors are connected with solar regions with the same polarity (Wilcox and Ness, 1965). From magnetic field in situ measurements is possible to infer the local HCS inclination. The most employed method for determining this inclination is the minimum variance analysis (MVA) that uses the eigenvector associated with the minimum eigenvalue as proxy of the normal vector to the HCS plane (Sonnerup and Cahill, 1967). Previous works applying MVA support that important variations of the local HCS inclination are not observed along
ARTICLE IN PRESS J.J. Blanco et al. / Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 226–233
227
the solar cycle. This result can be explained because of the presence of local waviness in the HCS (Villante et al., 1979; Behannon et al., 1981). The HCS is embedded in a solar wind region characterized by high density, low temperature, slow solar wind (SSW) velocity and high plasma beta called heliospheric plasma sheet (HPS) (Winterhalter et al., 1994). Recently, Crooker et al. (2004) warn about the existence of high plasma beta regions without HCS and HCS out of these regions. In this work we survey the local HCS properties from January 1995 to May 2001 focussing our attention on its local inclination and solar wind structures close to the HCS. 2. Methodology and data analysis A systematic search of HCS crossings detected by WIND spacecraft has been done from January 1995 to May 2001. We assume that a HCS crossing happens when a magnetic field minimum, ! ! showing !a polarity reversal is observed and Q e B (where Q e is the electron heat flux in solar wind) reverses its sign through an interval no longer than 60 min (Fig. 1). The latter can also be observed in the low energy electron pitch angle distribution (Kahler et al., 1998). We have used data from magnetic field instrument (MFI) (Lepping et al., 1995) for the magnetic field, solar wind experiment (SWE) (Ogilvie et al., 1995) for solar wind (proton and electron) properties and 3-D plasma instrument for the electron pitch angle (Lin et al., 1995). Unfortunately, valid electron data from SWE are only available until May 2001. During the observed period the WIND spacecraft was at 1 AU and its orbit crossed the Magnetosphere many times. The intervals inside of Magnetosphere have been removed in our study. Each HCS crossing has been grouped into categories depending on the solar wind conditions measured in the HCS crossing neighbourhood. Three categories have been established: SSW if the solar wind velocity is lower than 450 km/s and it is far from transition regions between fast solar wind and SSW, stream interaction region (SIR) if the solar wind velocity grows quickly developing an interaction region and magnetic cloud (MC) if a structure in the solar wind shows a smooth rotation of the magnetic field, simultaneously to a low temperature and a relatively high magnetic field strength (Burlaga et al., 1981). Once time an event has been grouped in one of the previous categories the mean density and temperature of
Fig. 1. Interplanetary magnetic field and electron data detected by WIND spacecraft at 1 AU on March 9, 1995. From top to bottom: 320 eV electron pitch angle, QeB sign, GSE magnetic field components and magnetic field strength. Red represents the greatest intensity and blue represents zero intensity in the color scale used in the electron pitch angle graph.
Fig. 2. HCS crossing detected by WIND spacecraft sensors on March 9, 1995. Squares, circles and triangles are the Bx, By and Bz magnetic field components respectively. Continuous lines and the values show in the left panel are the fit lines, the reduced CHIsquare w2 , the coefficient of determination R2 , and the parameters obtained using Eq. (2) as fit function.
electrons and protons, solar wind velocity and local inclination of the HCS crossings have been calculated in an interval 2 times longer than the HCS width. If multiple HCS crossings are detected, they
ARTICLE IN PRESS J.J. Blanco et al. / Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 226–233
228
y y
are considered as a single one and the mean values estimated in the region containing the multiple crossings are used for solar wind properties and HCS local inclination. We have catalogued two or more consecutive HCS crossings as a multiple crossing when the interval between two consecutive events is less than 30 min (Blanco et al., 2006). Around 10% of the HCS crossings have been considered as multiple one. The HCS local inclination is estimated using the method described inBlanco et al. (2003). This method, based on a modified Harris field, assumes that the HCS is plane locally and the magnetic field, in a local system where the y direction coincides with the vector perpendicular to the plane and the other two are in the HCS plane, can be written as: y y 0 Bx ¼ Bx0 tanh ; By ¼ Byo ; Bz ¼ Bz0 . L (1)
By ¼ Bx0 tanh
Where L is the HCS semi width and y0 marks the point where the magnetic field reversal happens. Bx0 , By0 and Bz0 are the magnetic field components just out of the HCS plane. Nevertheless, when a spacecraft crosses the HCS its magnetometer detects a magnetic structure that it should be the previous one (Eq. (1)) but rotated with respect the spacecraft trajectory (see Fig. 1 in Blanco et al., 2003). If our initial assumption about the local shape of the HCS is correct, only two consecutive rotations applied to Eq. (1) are necessary for obtaining the magnetic configuration measured by the spacecraft magnetometer. Considering a first rotation around the z axis by a a angle and a second one around the x0 axis, i.e. the rotated x axis, by a b angle the Eq. (1) can be written as: y y 0 Bx ¼ Bx0 tanh (2) cos a By0 sin a, L y y 0 By ¼ Bx0 tanh sin a cos b þ By0 cos a cos b L ð3Þ Bz0 sin b,
3. Results: HCS crossings and solar wind
0
L þ Bz0 cos b.
sin a sin b þ By0 cos a sin b ð4Þ
Using this magnetic field as fit function to magnetic data the normal vector to the HCS plane can be estimated from: ! n ¼ ð sin a; cos a sin b; cos a sin bÞ. (5) The used fit procedure is based on the minimization of the CHI-square function where the theoretical function is the rotated Harris field and Levenberg– Marquardt is the employed algorithm. As for the selected data interval, it has to be greater than the HCS crossing width because our method needs to test the interplanetary magnetic field conditions close to both sides of HCS. An example of this procedure is shown in Fig. 2.
A total of 165 HCS crossings have been considered, 71 were in SSW regions, 67 close to SIR and 27 were associated with a MC passage. The results have been summarized in Table 1. The solar wind density in SIR and MC is higher than in SSW, probably related with the presence of high pressure regions. The highest mean velocity is associated with MC events. Here we have to point out that the SIR events are located closer to the SSW stream for what it should not be rare to find the previous result. Electron and proton temperatures are quite different by almost an order in magnitude, as it could be expected for two particle populations that do not interact. The HCS width is calculated from the time of crossing, the normal vector to the HCS and the solar wind velocity. The obtained values are closer to those found by Winterhalter et al. (1994) for the HPS than the HCS. In this point, a brief commentary on how the values were obtained in
Table 1 Resume of heliospheric current sheet crossings grouped by their associated solar wind phenomena Solar wind
Events
N e ðe=cm3 Þ
N p ðp=cm3 Þ
T e (K)
T p (K)
V SW (km/s)
f (1)
y (1)
L (km)
SSW SIR MC All
71 67 27 165
11 1 19 2 16 2 15 1
12 1 20 2 16 2 16 1
118 000 4000 150 000 5000 110 000 5000 130 000 3000
30 000 2000 43 000 4000 41 000 5000 37 000 3000
366 6 393 9 400 10 382 5
28 3 29 3 56 25 2
17 2 14 2 18 3 16 1
600 000 40 000 1 300 000 1 200 000 320 000 80 000 900 000 500 000
Slow solar wind (SSW), stream interaction region (SIR) and magnetic cloud (MC). The columns show the mean values and the error of the mean of electron density, proton density, electron temperature, proton temperature, solar wind speed, longitude and elevation angles of the normal vector to the HCS plane and heliospheric current sheet width.
ARTICLE IN PRESS J.J. Blanco et al. / Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 226–233
Table 1 is needed. Each quantity is the mean value and the uncertainty is the statistical error. For all the magnitudes, the uncertainty is reasonably low but this is not truth for the HCS width. In our opinion, this can be due mainly to two factors: the existence of nine extremely wide crossings ð41 106 kmÞ and an overestimation of the time of crossing for these HCS crossings. As for the HCS local orientation, the longitude angle of the normal vector to the HCS plane lies closer to the ortho-spiral (i.e. 45 ) in SIR than SSW according with a greater solar wind velocity. For MC events, this angle is away from the ortho-spiral. This can be due to an important effect on the HCS shape of the MC propagation. The HCS deformation is clearly observed in Fig. 3. Histograms of the longitude and elevation angles are presented for SSW, SIR and MC. In this figure we observe how the HCS associated with MC show the highest dispersion in the longitude angle while that for SIR and SSW this angle
229
is around 45 and 30 respectively. As for the elevation angle, the three categories show a quasiperpendicular HCS local inclination, nevertheless, the HCS crossings close to MCs seem to be uniform until 30 and those associated with SIRs are concentrated in the range of 0 –10 . We interpret these results with a strong HCS dynamic deformation due to the MC and SIR presence. 4. Results: HCS along the solar cycle The studied period begins at the last part of descending phase of cycle 22 (1995) and finishes at the beginning of descending phase of cycle 23 (2001). In 1996 an extremely low number of HCS crossings were detected (12 events) and in 2001 HCS crossings were only considered until May because of the electron detector of SWE left to work at that time.
Fig. 3. Histograms of the longitude (upper row) and the elevation (down row) angles of the normal vector to the HCS plane. Events have been grouped in three categories: SSW events first column, SIR events second column and MC events third column.
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All the considered HCS crossings have been grouped by year and categories in Table 2. It is clear that the solar wind conditions around the HCS crossings vary along this period (Fig. 4). Around 1998, a minimum in solar wind density appears while a minimum and a maximum in speed are observed in 1996 and 1999 respectively. The temperature follows the speed trends throughout the whole period. Nevertheless, the electron temperature
Table 2 Number of heliospheric current sheet crossings grouped by year and categories Year Slow solar wind (SSW)
Stream interaction Magnetic HCS region (SIR) cloud (MC) crossings
1995 1996 1997 1998 1999 2000 2001
14 4 4 8 14 16 7
10 4 15 25 5 8 4
5 4 6 6 2 3 1
29 12 25 39 21 27 12
seems to be slight shifted with respect to the proton temperature. In 1995 during the last stage of solar cycle 22, the solar wind conditions measured near HCS crossings seem to be balancing between SSW regions and SIRs. In 1997 and 1998, in coincidence with the initial ascending phase of solar cycle 23, the HCS crossings were majority detected in SSW regions. During these years the number of HCS crossings associated with MCs was higher than that associated with SIRs. This trend changes in 1999 and 2000 where the dominant conditions near the HCS crossings were those of SIR. The local orientation of the HCS does not seem to have a clear dependence on the solar cycle (Figs. 5 and 6). This is not in contradiction with a global shape of the HCS which evolves along the solar cycle if assuming a local waviness (Villante et al., 1979; Behannon et al., 1981). The HCS is mainly oriented along to the nominal Parker spiral during the whole period. ‘‘Pure’’ signatures, i.e. SSW, SIR or MC, are not observed in any year but in 1996 (Fig. 5). Higher elevation angle were estimated in coincidence with a higher relative number of MC events (Fig. 6 and Table 2). Our results seem to corroborate a local HCS orientation depending on the ratio among SSW, SIR and MC events more than solar cycle dependence. 5. Conclusions In this work we have followed the HCS local structure from January 1995 to May 2001. After rejecting intervals inside magnetosphere, data gaps and crossings showing highly fluctuating magnetic fields a total of 165 HCS crossings have been considered. They have been grouped into three categories depending on the observed solar wind conditions during the crossings. These categories were SSW, SIR or MC. Our results can be summarized in the following points:
Fig. 4. Annual mean values of solar wind in HCS crossings. From top to bottom: HCS width, solar wind velocity, proton temperature, electron temperature, proton density and electron density. The error bars are the statistical errors.
The density of protons and electron in crossings near MCs and SIRs is higher than SSW. This fact is probably due to the presence of high pressure regions. There seem to be strong deformation on the HCS local shape when it is detected close to MCs. Otherwise; SIRs seem to push the HCS local plane towards quasi-perpendicular elevation angles. Both phenomena exert a strong dynamic deformation on the HCS local structure.
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Fig. 5. Annual histograms of the longitude angle of the normal vector to the HCS plane.
During the early ascending phase of solar cycle 23 (1997–1998) the HCS is mainly located in SSW regions. This trend changes in 1999 and 2000 when the HCS is principally detected near SIRs. The solar wind characteristics change along the studied period showing a density minimum around 1998 and a velocity minimum in 1996 and a maximum in 1999. The local HCS inclination does not seem to be directly related to the solar cycle but the relative
number of MC, SIR and SSW. Nevertheless, the HCS is mainly oriented along the Parker Spiral.
The studied period (6.5 years) is too short for establishing conclusive results about the HCS local structure along the solar cycle. We will extend our study to the whole solar cycle and compare with the available data of previous cycles with the goal to confirm or not the results presented in this work.
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Fig. 6. Annual histograms of the elevation angle of the normal vector to the HCS plane.
Acknowledgements
References
The authors wish to thank the WIND/MFI, WIND/SWE and WIND/3DP teams for the use of their data and especially to Dr A. Vin˜as for his comments on electrons in the solar wind. This work has been supported by Spanish Ministerio de Eduacio´n y Ciencia into the project with reference code: ESP2005-07290-C02-01 and by Universidad de Alcala´ into the project with reference code: ESP2006-08459/.
Bavassano, B., Woo, R., Bruno, R., 1997. Heliospheric plasma sheet and coronal streamers. Geophysical Research Letters 24 (13), 1655–1658. Behannon, K.W., Neubauer, F.M., Barnstorf, H., 1981. Finescale characteristics of interplanetary sector boundaries. Journal of Geophysical Research 86, 3273. Blanco, J.J., Rodrıguez-Pacheco, J., Sequeiros, J., 2003. A new method for determining the interplanetary current-sheet local orientation. Solar Physics 213, 147. Blanco, J.J., Rodrıguez-Pacheco, J., Hidalgo, M.A., Sequeiros, J., 2006. Analysis of the heliospheric current sheet fine structure:
ARTICLE IN PRESS J.J. Blanco et al. / Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 226–233 single or multiple current sheets. Journal of Atmospheric and Terrestrial Physics 68, 2173–2181. Burlaga, L.F., Sittler, E., Mariani, F., Schwenn, R., 1981. Magnetic loop behind an interplanetary shock: Voyager, Helios, and IMP8 observations. Journal of Geophysical Research 86, 6673–6684. Crooker, N.U., Huang, C., Lamassa, S.M., Larson, D.E., Kahler, S.W., Spence, H.E., 2004. Heliospheric plasma sheets. Journal of Geophysical Research 109, 3107. Kahler, S., Crooker, N.U., Gosling, J.T., 1998. Properties of interplanetary magnetic sector boundaries based on electron heat-flux flow directions. Journal of Geophysical Research 103, 20603. Lepping, R.P., Acuna, M., Burlaga, L., Farrell, W., Slavin, J., Schatten, K., Mariani, F., Ness, N., Neubauer, F., Whang, Y.C., Byrnes, J., Kennon, R., Panetta, P., Scheifele, J., Worley, E., 1995. The WIND magnetic field investigation. Space Science Reviews 71, 207. Lin, R.P., et al., 1995. Large-dimensional plasma and energetic particle investigation for the WIND spacecraft. Space Science Reviews 71, 125. Ogilvie, K.W., Chorney, D.J., Fitzenreiter, R.J., Hunsaker, F., Keller, J., Lobell, J., Miller, G., Scudder, J.D., Sittler Jr., E.C., Torbert, R.B., Bodet, D., Needell, G., Lazarus, A.J.,
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Steinberg, J.T., Tappan, J.H., Mavretic, A., Gergin, E., 1995. SWE, a comprehensive plasma instrument for the WIND spacecraft. Space Science Reviews 71, 55. Riley, P., Linker, J.A., Mikic, Z., 2002. Modeling the heliospheric current sheet solar cycle variations. Journal of Geophysical Research 107. Smith, E.J., 2001. The heliospheric current sheet. Journal of Geophysical Research 106, 15819. Sonnerup, B.U.O¨., Cahill, L.J., 1967. Magnetopause structure and attitude from explorer 12 observations. Journal of Geophysical Research 72, 171. Villante, U., Bruno, R., Mariani, F., Burlaga, L.F., Ness, N.F., 1979. The shape and location of the sector boundary surface in the inner solar system. Journal of Geophysical Research 84, 6641. Wang, Y.-M., Sheeley, N.R., Rich, N.B., 2000. Evolution of coronal streamer structure during the rising phase of solar cycle 23. Geophysical Research Letters 27, 149. Wilcox, J.M., Ness, N.F., 1965. Quasi-stationary corotating structure in the interplanetary medium. Journal of Geophysical Research 70, 5792. Winterhalter, D., Smith, E.J., Burton, M.E., Murphy, N., McComas, D.J., 1994. The heliospheric plasma sheet. Journal of Geophysical Research 99, 6667.
Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 226–233 www.elsevier.com/locate/jastp
Monitoring the heliospheric current sheet local structure for the years 1995 to 2001 J.J. Blanco, J. Rodriguez-Pacheco, M.A. Hidalgo, J. Sequeiros Space Research Group, Dpto. Fisica, Univ. Alcala, Alcala de Henares-Madrid, Spain Accepted 27 August 2007 Available online 1 October 2007
Abstract We have monitored the heliospheric current sheet (HCS) local structure through a 6.5 year period starting in January 1995. This interval begins with near solar minimum conditions and finishes in solar maximum conditions. We have used data from Wind mission, mainly the magnetic field instrument (MFI) and the solar wind experiment (SWE). Our work is focused on the HCS local inclination and the solar wind conditions around the HCS crossings, with a particular interest on their evolution along the ascending phase of solar cycle 23 and its relationship with solar wind phenomena such as magnetic cloud and stream interaction regions. We defined a real HCS crossing when a magnetic field minimum, showing a polarity reversal is observed and QeB (where Qe is the electron heat flux in solar wind) reverses its sign through an interval no longer than 60 min. The results suggest that the HCS local structure is more dependent on the solar wind conditions than on the solar cycle stage. r 2007 Elsevier Ltd. All rights reserved. Keywords: Heliospheric current sheet; Solar cycle; Stream interaction region; Magnetic cloud; Slow solar wind
1. Introduction The heliospheric current sheet (HCS) is probably the most relevant feature immersed the solar wind. It is present along all the solar cycle although its global shape suffers important changes, particularly close to the solar maximum (Smith, 2001; Riley et al., 2002). The HCS goes into the heliosphere from the cusp of the magnetic arcs that form the helmet streamer belt. So any change in the helmet streamer should affect to the global shape of the HCS, this is clear during the solar maximum when Corresponding author. Tel.: +34 91 8855053; fax: +34 91 8854942. E-mail address: [email protected] (J.J. Blanco).
1364-6826/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2007.08.030
the helmet streamer fills wide corona regions (Bavassano et al., 1997; Wang et al., 2000). At 1 AU the HCS is observed as a sharp boundary between regions with opposite magnetic polarity dividing the heliosphere in magnetic sectors. These sectors are connected with solar regions with the same polarity (Wilcox and Ness, 1965). From magnetic field in situ measurements is possible to infer the local HCS inclination. The most employed method for determining this inclination is the minimum variance analysis (MVA) that uses the eigenvector associated with the minimum eigenvalue as proxy of the normal vector to the HCS plane (Sonnerup and Cahill, 1967). Previous works applying MVA support that important variations of the local HCS inclination are not observed along
ARTICLE IN PRESS J.J. Blanco et al. / Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 226–233
227
the solar cycle. This result can be explained because of the presence of local waviness in the HCS (Villante et al., 1979; Behannon et al., 1981). The HCS is embedded in a solar wind region characterized by high density, low temperature, slow solar wind (SSW) velocity and high plasma beta called heliospheric plasma sheet (HPS) (Winterhalter et al., 1994). Recently, Crooker et al. (2004) warn about the existence of high plasma beta regions without HCS and HCS out of these regions. In this work we survey the local HCS properties from January 1995 to May 2001 focussing our attention on its local inclination and solar wind structures close to the HCS. 2. Methodology and data analysis A systematic search of HCS crossings detected by WIND spacecraft has been done from January 1995 to May 2001. We assume that a HCS crossing happens when a magnetic field minimum, ! ! showing !a polarity reversal is observed and Q e B (where Q e is the electron heat flux in solar wind) reverses its sign through an interval no longer than 60 min (Fig. 1). The latter can also be observed in the low energy electron pitch angle distribution (Kahler et al., 1998). We have used data from magnetic field instrument (MFI) (Lepping et al., 1995) for the magnetic field, solar wind experiment (SWE) (Ogilvie et al., 1995) for solar wind (proton and electron) properties and 3-D plasma instrument for the electron pitch angle (Lin et al., 1995). Unfortunately, valid electron data from SWE are only available until May 2001. During the observed period the WIND spacecraft was at 1 AU and its orbit crossed the Magnetosphere many times. The intervals inside of Magnetosphere have been removed in our study. Each HCS crossing has been grouped into categories depending on the solar wind conditions measured in the HCS crossing neighbourhood. Three categories have been established: SSW if the solar wind velocity is lower than 450 km/s and it is far from transition regions between fast solar wind and SSW, stream interaction region (SIR) if the solar wind velocity grows quickly developing an interaction region and magnetic cloud (MC) if a structure in the solar wind shows a smooth rotation of the magnetic field, simultaneously to a low temperature and a relatively high magnetic field strength (Burlaga et al., 1981). Once time an event has been grouped in one of the previous categories the mean density and temperature of
Fig. 1. Interplanetary magnetic field and electron data detected by WIND spacecraft at 1 AU on March 9, 1995. From top to bottom: 320 eV electron pitch angle, QeB sign, GSE magnetic field components and magnetic field strength. Red represents the greatest intensity and blue represents zero intensity in the color scale used in the electron pitch angle graph.
Fig. 2. HCS crossing detected by WIND spacecraft sensors on March 9, 1995. Squares, circles and triangles are the Bx, By and Bz magnetic field components respectively. Continuous lines and the values show in the left panel are the fit lines, the reduced CHIsquare w2 , the coefficient of determination R2 , and the parameters obtained using Eq. (2) as fit function.
electrons and protons, solar wind velocity and local inclination of the HCS crossings have been calculated in an interval 2 times longer than the HCS width. If multiple HCS crossings are detected, they
ARTICLE IN PRESS J.J. Blanco et al. / Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 226–233
228
y y
are considered as a single one and the mean values estimated in the region containing the multiple crossings are used for solar wind properties and HCS local inclination. We have catalogued two or more consecutive HCS crossings as a multiple crossing when the interval between two consecutive events is less than 30 min (Blanco et al., 2006). Around 10% of the HCS crossings have been considered as multiple one. The HCS local inclination is estimated using the method described inBlanco et al. (2003). This method, based on a modified Harris field, assumes that the HCS is plane locally and the magnetic field, in a local system where the y direction coincides with the vector perpendicular to the plane and the other two are in the HCS plane, can be written as: y y 0 Bx ¼ Bx0 tanh ; By ¼ Byo ; Bz ¼ Bz0 . L (1)
By ¼ Bx0 tanh
Where L is the HCS semi width and y0 marks the point where the magnetic field reversal happens. Bx0 , By0 and Bz0 are the magnetic field components just out of the HCS plane. Nevertheless, when a spacecraft crosses the HCS its magnetometer detects a magnetic structure that it should be the previous one (Eq. (1)) but rotated with respect the spacecraft trajectory (see Fig. 1 in Blanco et al., 2003). If our initial assumption about the local shape of the HCS is correct, only two consecutive rotations applied to Eq. (1) are necessary for obtaining the magnetic configuration measured by the spacecraft magnetometer. Considering a first rotation around the z axis by a a angle and a second one around the x0 axis, i.e. the rotated x axis, by a b angle the Eq. (1) can be written as: y y 0 Bx ¼ Bx0 tanh (2) cos a By0 sin a, L y y 0 By ¼ Bx0 tanh sin a cos b þ By0 cos a cos b L ð3Þ Bz0 sin b,
3. Results: HCS crossings and solar wind
0
L þ Bz0 cos b.
sin a sin b þ By0 cos a sin b ð4Þ
Using this magnetic field as fit function to magnetic data the normal vector to the HCS plane can be estimated from: ! n ¼ ð sin a; cos a sin b; cos a sin bÞ. (5) The used fit procedure is based on the minimization of the CHI-square function where the theoretical function is the rotated Harris field and Levenberg– Marquardt is the employed algorithm. As for the selected data interval, it has to be greater than the HCS crossing width because our method needs to test the interplanetary magnetic field conditions close to both sides of HCS. An example of this procedure is shown in Fig. 2.
A total of 165 HCS crossings have been considered, 71 were in SSW regions, 67 close to SIR and 27 were associated with a MC passage. The results have been summarized in Table 1. The solar wind density in SIR and MC is higher than in SSW, probably related with the presence of high pressure regions. The highest mean velocity is associated with MC events. Here we have to point out that the SIR events are located closer to the SSW stream for what it should not be rare to find the previous result. Electron and proton temperatures are quite different by almost an order in magnitude, as it could be expected for two particle populations that do not interact. The HCS width is calculated from the time of crossing, the normal vector to the HCS and the solar wind velocity. The obtained values are closer to those found by Winterhalter et al. (1994) for the HPS than the HCS. In this point, a brief commentary on how the values were obtained in
Table 1 Resume of heliospheric current sheet crossings grouped by their associated solar wind phenomena Solar wind
Events
N e ðe=cm3 Þ
N p ðp=cm3 Þ
T e (K)
T p (K)
V SW (km/s)
f (1)
y (1)
L (km)
SSW SIR MC All
71 67 27 165
11 1 19 2 16 2 15 1
12 1 20 2 16 2 16 1
118 000 4000 150 000 5000 110 000 5000 130 000 3000
30 000 2000 43 000 4000 41 000 5000 37 000 3000
366 6 393 9 400 10 382 5
28 3 29 3 56 25 2
17 2 14 2 18 3 16 1
600 000 40 000 1 300 000 1 200 000 320 000 80 000 900 000 500 000
Slow solar wind (SSW), stream interaction region (SIR) and magnetic cloud (MC). The columns show the mean values and the error of the mean of electron density, proton density, electron temperature, proton temperature, solar wind speed, longitude and elevation angles of the normal vector to the HCS plane and heliospheric current sheet width.
ARTICLE IN PRESS J.J. Blanco et al. / Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 226–233
Table 1 is needed. Each quantity is the mean value and the uncertainty is the statistical error. For all the magnitudes, the uncertainty is reasonably low but this is not truth for the HCS width. In our opinion, this can be due mainly to two factors: the existence of nine extremely wide crossings ð41 106 kmÞ and an overestimation of the time of crossing for these HCS crossings. As for the HCS local orientation, the longitude angle of the normal vector to the HCS plane lies closer to the ortho-spiral (i.e. 45 ) in SIR than SSW according with a greater solar wind velocity. For MC events, this angle is away from the ortho-spiral. This can be due to an important effect on the HCS shape of the MC propagation. The HCS deformation is clearly observed in Fig. 3. Histograms of the longitude and elevation angles are presented for SSW, SIR and MC. In this figure we observe how the HCS associated with MC show the highest dispersion in the longitude angle while that for SIR and SSW this angle
229
is around 45 and 30 respectively. As for the elevation angle, the three categories show a quasiperpendicular HCS local inclination, nevertheless, the HCS crossings close to MCs seem to be uniform until 30 and those associated with SIRs are concentrated in the range of 0 –10 . We interpret these results with a strong HCS dynamic deformation due to the MC and SIR presence. 4. Results: HCS along the solar cycle The studied period begins at the last part of descending phase of cycle 22 (1995) and finishes at the beginning of descending phase of cycle 23 (2001). In 1996 an extremely low number of HCS crossings were detected (12 events) and in 2001 HCS crossings were only considered until May because of the electron detector of SWE left to work at that time.
Fig. 3. Histograms of the longitude (upper row) and the elevation (down row) angles of the normal vector to the HCS plane. Events have been grouped in three categories: SSW events first column, SIR events second column and MC events third column.
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All the considered HCS crossings have been grouped by year and categories in Table 2. It is clear that the solar wind conditions around the HCS crossings vary along this period (Fig. 4). Around 1998, a minimum in solar wind density appears while a minimum and a maximum in speed are observed in 1996 and 1999 respectively. The temperature follows the speed trends throughout the whole period. Nevertheless, the electron temperature
Table 2 Number of heliospheric current sheet crossings grouped by year and categories Year Slow solar wind (SSW)
Stream interaction Magnetic HCS region (SIR) cloud (MC) crossings
1995 1996 1997 1998 1999 2000 2001
14 4 4 8 14 16 7
10 4 15 25 5 8 4
5 4 6 6 2 3 1
29 12 25 39 21 27 12
seems to be slight shifted with respect to the proton temperature. In 1995 during the last stage of solar cycle 22, the solar wind conditions measured near HCS crossings seem to be balancing between SSW regions and SIRs. In 1997 and 1998, in coincidence with the initial ascending phase of solar cycle 23, the HCS crossings were majority detected in SSW regions. During these years the number of HCS crossings associated with MCs was higher than that associated with SIRs. This trend changes in 1999 and 2000 where the dominant conditions near the HCS crossings were those of SIR. The local orientation of the HCS does not seem to have a clear dependence on the solar cycle (Figs. 5 and 6). This is not in contradiction with a global shape of the HCS which evolves along the solar cycle if assuming a local waviness (Villante et al., 1979; Behannon et al., 1981). The HCS is mainly oriented along to the nominal Parker spiral during the whole period. ‘‘Pure’’ signatures, i.e. SSW, SIR or MC, are not observed in any year but in 1996 (Fig. 5). Higher elevation angle were estimated in coincidence with a higher relative number of MC events (Fig. 6 and Table 2). Our results seem to corroborate a local HCS orientation depending on the ratio among SSW, SIR and MC events more than solar cycle dependence. 5. Conclusions In this work we have followed the HCS local structure from January 1995 to May 2001. After rejecting intervals inside magnetosphere, data gaps and crossings showing highly fluctuating magnetic fields a total of 165 HCS crossings have been considered. They have been grouped into three categories depending on the observed solar wind conditions during the crossings. These categories were SSW, SIR or MC. Our results can be summarized in the following points:
Fig. 4. Annual mean values of solar wind in HCS crossings. From top to bottom: HCS width, solar wind velocity, proton temperature, electron temperature, proton density and electron density. The error bars are the statistical errors.
The density of protons and electron in crossings near MCs and SIRs is higher than SSW. This fact is probably due to the presence of high pressure regions. There seem to be strong deformation on the HCS local shape when it is detected close to MCs. Otherwise; SIRs seem to push the HCS local plane towards quasi-perpendicular elevation angles. Both phenomena exert a strong dynamic deformation on the HCS local structure.
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Fig. 5. Annual histograms of the longitude angle of the normal vector to the HCS plane.
During the early ascending phase of solar cycle 23 (1997–1998) the HCS is mainly located in SSW regions. This trend changes in 1999 and 2000 when the HCS is principally detected near SIRs. The solar wind characteristics change along the studied period showing a density minimum around 1998 and a velocity minimum in 1996 and a maximum in 1999. The local HCS inclination does not seem to be directly related to the solar cycle but the relative
number of MC, SIR and SSW. Nevertheless, the HCS is mainly oriented along the Parker Spiral.
The studied period (6.5 years) is too short for establishing conclusive results about the HCS local structure along the solar cycle. We will extend our study to the whole solar cycle and compare with the available data of previous cycles with the goal to confirm or not the results presented in this work.
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Fig. 6. Annual histograms of the elevation angle of the normal vector to the HCS plane.
Acknowledgements
References
The authors wish to thank the WIND/MFI, WIND/SWE and WIND/3DP teams for the use of their data and especially to Dr A. Vin˜as for his comments on electrons in the solar wind. This work has been supported by Spanish Ministerio de Eduacio´n y Ciencia into the project with reference code: ESP2005-07290-C02-01 and by Universidad de Alcala´ into the project with reference code: ESP2006-08459/.
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