A study has been made of the correlation of the population of relativistic electrons in the outer-zone magnetosphere with the properties of the solar wind (speed, density, magnetic field) during a solar minimum period. The study is based upon observations made in the Spring of 1995 with sensors aboard 1994-026 and WIND. It is found that a large relativistic electron enhancement depends upon a substantial solar-wind speed increase associated with precursor solar-wind density enhancement, and, in particular, upon a southward turning of the interplanetary magnetic fiel

Baker, D N

Blake, J B

Lepping, R P

Ogilvie, K

Turner, Niescja E

English

Trinity University

Trinity University Digital Commons @ Trinity Physics and Astronomy Faculty Research Physics and Astronomy Department 1997 Correlation of Changes in the Outer-Zone Relativistic-Electron Population with Upstream Solar Wind Measurements J B. Blake D N. Baker Niescja E. Turner Trinity University, nturner1@trinity.edu K Ogilvie R P. Lepping Follow this and additional works at: https://digitalcommons.trinity.edu/physics_faculty  Part of the Astrophysics and Astronomy Commons Repository Citation Blake, J.B., Baker, D.N., Turner, N., Ogilvie, K.W., & Lepping, R.P. (1997). Correlation of changes in the outer-zone relativistic-electron population with upstream solar wind and magnetic field measurements. Geophysical Research Letters, 24(8), 927-929. doi: 10.1029/97GL00859 This Article is brought to you for free and open access by the Physics and Astronomy Department at Digital Commons @ Trinity. It has been accepted for inclusion in Physics and Astronomy Faculty Research by an authorized administrator of Digital Commons @ Trinity. For more information, please contact jcostanz@trinity.edu. GEOPHYSICAL RESEARCH LETTERS, VOL. 24, NO.8, PAGES 927-929, APRIL 15, 1997 Correlation of changes in the outer-zone relativistic-electron population with upstream solar wind and magnetic field measurements J. B. Blake • D. N. Baker 2, N. Turner 2, K. W Ogilvie 3, and R. P. Lepping 3 Abstract. A study has been made of the correlation of the population of relativistic electrons in the outer-zone magneto- sphere with the properties of the solar wind (speed, density, magnetic field) during a solar minimum period. The study is based upon observations made in the Spring of 1995 with sen- sors aboard 1994-026 and WIND. It is found that a large rela- tivistic electron enhancement depends upon a substantial solar- wind speed increase associated with precursor solar-wind den- sity enhancement, and, in particular, upon a southward turning of the interplanetary magnetic field. Introduction Over twenty years ago, it was discovered that the population of relativistic electrons in geostationary orbit (L = 6.6) was clearly modulated with the 27-day synodic period of the Sun. This modulation was most evident around the time of solar minimum, and the modulation was driven in some unknown fashion by high-speed solar-wind streams (Paulikas and Blake 1976, 1979; Baker et al. 1978). Additional studies have been carried out subsequently (Baker et al. 1986, 1994), but these analyses have been made in the absence of comprehensive measurements of the highly time variable solar wind imping- ing upon the Earth's magnetosphere. With the launch of the WIND spacecraft, the necessary measurements now are rou- tinely available. In this Letter, we present a first report on the continuous correlation of solar-wind properties with the state of the relativistic electron population in the Earth's magnetosphere. Instrumentation and Spacecraft WIND provides a continuous record of solar-wind speed and density, and of the interplanetary magnetic field (Ogilvie et al. 1995, Lepping et al. 1995), except for the rare occasions when WIND passes through the magnetosphere near perigee. Space- craft 1994-026 (-62 ø inclination with the argument of perigee fixed at 270 ø, 1.15 X 7.2 R E orbit, = 12-h period) measures the outer-zone relativistic electron population in four outer-zone passages per day with four integral energy channels from > 1.5 MeV to > 8.5 MeV). The s/c covers all L values from 2 to beyond the trapping boundary every orbit. Orbit-integrated doses (which are directly related to orbit-integrated electron •Space Sciences Department, The Aerospace Corporation, Los Angeles California. 2Laboratory for Atmospheric and Space Physics, University of Colo- rado, Boulder, Colorado. 3NASA/Goddard Space Flight Center, Greenbelt, Maryland. Copyright 1997 by the American Geophysical Union. Paper number 97GL00859. 0094-8534/97/97GL-00859505.00 fluences) are a concise way to view the electron population above a threshold energy averaged over an entire orbit. We are interested here in global effects--the behavior of the outer zone electron population as a single system. The inbound and outbound passes of 1994-026 in the radiation belt are tempo- rally close together; therefore, the two observations are summed and a 12-h value for the electron environment is derived. The focus has been upon measurements made in early 1995, when, for several successive solar rotations, high-speed solar-wind streams strongly modulated the relativistic electron population in the outer zone, and when WIND data are available. '• 100 1.5 MeV {76 n/tq)E > 3_5 MeV (267 mitq) ! ! I i I I I I t 400 420 440 460 480 500 520 540 Figure 1. The upper panel shows the time history of outer- zone relativistic electron population during a 160-day period in the Spring of 1995. The plot is in units of dose per 12-hour orbit; the two channels are labeled by both the electron thresh- old energy and nominal electron range in aluminum. The bot- tom panel shows the solar-wind speed from WIND and the radia- tion intensity from 1994-026 plotted for a portion of the interval shown in the top panel. 927 928 BLAKE ET AL.: CORRELATION OF RELATIVISTIC ELECTRONS WITH SOLAR WIND Observations 7110 Figure la shows the run of radiation belt data for approxi- mately a 160-day period during the first half of 1995. Two instrumental features should be noted. First, the oscillatory o• 600,•• behavior seen in the t mporal sequence is d  to the fa t that the magnetic latitude (B/B0) varies sy tematically from ne 'orbit to the next because of tilt of the Earth's dipole with respect to thes/c orbital pl ne. The synchronism between th  orbital period and the rotational period of the Earth ensures that this ntensity modulation is maintained. A high time variabil- ity and clear periodicity are present. Just prior to the large increases, it is common to see a brief, large decrease. The rise I •00 time in the >3.5-MeV lectron channel is noticeably longer 4811 485 q90 495 than that in the >I.5-MeV channel, whereas the fall times are comparable. In Figure lb the solar-wind speed from WIND and the radia- tion-belt data from 1994-026 are plotted for the central time period of the interval shown in Figure l a. The five large, abrupt increases in solar-wind speed are labeled A through E. It can be seen that two of the increases, A and D, were of the type described earlier (Paulikas and Blake 1979, Baker et al. 1994); at onset, the radiation intensity dropped substantially for a brief period (-12 h), followed by a more gradual, energy- dependent increase. Cases B and E also show a radiation decrease near the time of the onset of the high-speed wind. 1800 700 600 •oo • 400 • •00 2OO 47O 10 -15 : : ! : '"" : 2n 111t i i ! i ! i J i - •'60 't62 •16q •166 468 Day Nmnber (940101=1) 1ooo 470 Figure 2. The data from Fig. 1 are plotted in the top panel on an expanded time scale for the Case A time period. In the bottom panel, the quantity p.•wV.,w is plotted, suitably scaled, as well as the z component of the interplanetary magnetic field. 12 6 -6 -12 480 485 490 495 Day Number (940101=1) Figure 3. The two panels are similar to those in Fig. 2, showing the observations for Cases C and D. However, associated with Case B, there was no intensity increase, and for Case E, a modest increase was seen only in the >I.5-MeV channel. However, in both Cases, B and E, the increase in solar-wind speed was relatively small. Case C is quite different from the others. The solar-wind speed increased to a high value without an initial decrease and subsequent increase in radiation intensity. Figure 2a shows the time period around Case A on an expanded timescale; Figure 2b shows p.•wV.•w 2, suitably scaled, and the z-component of the interplanetary magnetic field. The pressure pulse ahead of, and during, the rise in solar-wind speed and the southward turning of the interplanetary field appear as precursors of the loss of the energetic electrons. Figure 3a,b is similar for Cases C and D in that a pressure pulse leads the solar-wind speed increase, but the response of the energetic electrons is quite different. In particular, in Case C, there is no observable effect of the high-speed solar wind upon the popula- tion of energetic electrons. However, the interplanetary field turned northward in Case C rather than southward as in Case D (and Case A). Discussion The five cases plotted in Figure lb may be surmarized as follows. Case A: There was a large increase in the solar-wind veloc- ity, a large pressure pulse, and a southward turning of the inter- BLAKE ET AL.: CORRELATION OF RELATIVISTIC ELECTRONS WITH SOLAR WIND 929 planetary field. The relativistic electrons first strongly decreased over a few hours, and then increased over the next several days to a level much higher than prior to the arrival of the high-speed solar wind. Case B: Similar to Case A except that the solar-wind speed increase was much smaller. It appears to have been inadequate to cause a large increase in the relativistic electron intensity, although an initial rapid decrease was seen. Case C: The solar-wind increase was much greater than that in Case B, and the interplanetary field turned northward rather than southward. Relativistic electrons were neither lost nor accelerated. Case D: This case is similar to Case A. Case E: This event is similar to Case B, although the solar- wind velocity was somewhat larger. The relativistic electrons showed an obvious increase following the decrease. Over a dozen examples were observed in the Spring of 1995. What we have shown here is representative, with the cause and effect relat!onship being clearer when there is large increase in the solar-wind speed followed by a several day period without a subsequent increase in solar-wind speed. Isolated events avoid the complexity of multiple factors. The southward turning of the interplanetary magnetic field seems to be a key factor. If the interplanetary magnetic field turns southward, a high-speed stream and leading pressure pulse have a strong effect on the energetic electron .population in the Earth's magnetosphere. If the field turns clearly northward, the effect of the high-speed stream is nil. It has been argued that the southward turning of the magnetic field is associated with substorm-injected elec- trons in the energy range from a few tens to a few hundred keV (Baker et al. 1996) that can serve as a seed population for sub- sequent acceleration to relativistic energies. However elec- trons are present that could be further accelerated whether or not the field turns southward, cf. Case C. It seems that the process is more complex than simply providing an energetic electron seed population by means of a substorm(s). Note also that the loss process can occur without subsequent acceleration of elec- trons to relativistic energies. The brief period of decreased fluxes (the electron loss) suggests perhaps an adiabatic process engendered by the pressure pulse; the limited data from 1994- 026 precludes an investigation of this surmise. It appears that for Space Weather purposes, measurements of the solar-wind speed and density, along with the interplanetary magnetic field would permit forecasting near solar minimum the overall intensity of the outer-zone relativistic electron population. Acknowledgements. This work was supported at the Aerospace Corporation under NASA Cooperative Agreement 26979B, and at the University of Colorado by NAG5-2635. References resolution energetic particle measurements a  6.6 R E, 3, Low-energy electron anisotropies and short-term substorm predictions, J. Geo- phys. Res., 83, 4863, 1978. Baker, D. N., J. B. Blake, R. W. Klebesadel, and P. R. Higbie, Highly relativistic electrons in the Earth's outer magnetosphere, I. Lifetimes and temporal history 1979-1984, J. Geophys. Res., 91, 4265, 1986. Baker, D. N., J. B. Blake, L. B. Callis, J. R. Cummings, D. Hovestadt, S. Kanekal, B. Klecker, R. A. Mewaldt, and R. D. Zwickl, Relativistic Electron Acceleration and Decay Time Scales in the Inner and Outer Radiation Belts, Geophys. Res. Lett. 21,409, 1994. Baker, D. N., X. Li, J. B. Blake, and S. Kanekal, Strong electron accel- eration ith the Earth's magnetosphere, Advances in Space Science, in press, 1996. Lepping, R. P., M. H. Acufia, L. F. Burlaga, W. M. Farrell, J. A. Slavin, K. H. Schatten, F. Mariani, N. F. Ness, F. M. Neubauer, Y. C. Whang, J. B. Byrnes, R. S. Kennon, P. V. Panetta, J. Scheifele, and E. M. Worley, The WIND Magnetic Field Investigation, Space Science Reviews 71,207, 1995. Ogilvie, K. W., D. J. Chornay, R. J. Fritzenreiter, F. Hunsaker, J. Keller, J. Lobell, G. Miller, J. D. Scudder, E. C. Sittler Jr., R. B. Torbert, D. Bodet, G. Needell, A. J. Lazarus, J. T. Steinberg, J. H. Tappan, A. Mavretic and E. Gergin, SWE, A Comprehensive Plasma Instrument for the WIND spacecraft, Space Science Reviews 71, 55, 1995. Paulikas, G. A., and J. B. Blake, Modulation of trapped energetic elec- trons at 6.6 R E by the direction of the interplanetary magnetic field, Geophys. Res. Lett. 3, 227, 1976. Paulikas, G. A., and J. B. Blake, Effect of the Solar Wind on Magneto- spheric Dynamics: Energetic Electrons at the Synchronous Orbit, Quantitative Modeling of Magnetospheric Processes, ed. W. P. Olsen, American Geophys. Union, Washington, D.C., 180, 1979. J. B. Blake, Space Sciences Department, The Aerospace Corpora- tion, P.O. Box 92957, MS: M2/259, Los Angeles CA 90009-2957. (e- mail: blake @ dirac2.dnet.nasa.gov) D. N. Baker, Laboratory for Atmospheric and Space Physics, Uni- versity of Colorado, Boulder, CO. (e-mail: baker@lynx.colorado.edu) N. Turner, Laboratory for Atmospheric and Space Physics, Univer- sit3• of Colorado, Boulder, CO. (e-mail: turner@kitron.colorado.edu) K. W. Ogilvie, NASA/Goddard Space Flight Center, Greenbelt, MD. (e-mail: u2kwo @lepvax.gsfc.nasa.gov) R. P. Lepping, NASA/Goddard Space Flight Center, Greenbelt, MD. (e-mail: u5rpl @ lepvax.gsfc.nasa. gov) (Received October 14, 1996; revised January 7, 1997; accepted February 24, 1997.) 

Correlation of Changes in the Outer-Zone Relativistic-Electron Population with Upstream Solar Wind Measurements

https://digitalcommons.trinity.edu/cgi/viewcontent.cgi?article=1004&context=physics_faculty

Correlation of Changes in the Outer-Zone Relativistic-Electron Population with Upstream Solar Wind Measurements

Abstract

Similar works

Full text

Available Versions

Trinity University