Characteristic plasma properties during dispersionless substorm injections at geosynchronous orbit

Abstract

Abstract. The substorm-associated behavior of the thermal plasma (30eV < E < 40keV) in the plasma sheet is examined by means of a superposed epoch analysis, using a full year of data from a spacecraft in geosynchronous orbit. The zero epoch time is taken to be substom onset as indicated by a dispersionless energetic particle injection observed on the same spacecraft. Five classes of injection events are found to be well ordered by their average local times. These range from pure ion injections ~3 hours prior to local midnight, to ion injections followed a few minutes later by an electron injection ~2 hours before midnight, to simultaneous ion and electron injections close to midnight, to electron injections that are followed by an ion injection ~ 1 hour postmidnight, and finally to pure electron injections ~2 hours postmidnight. The thermal electrons show a significant increase in temperature (from ~ 1 to ~2 keV) and pressure at substorm onset, while the density and the thermal ion signatures (below ~30 keV) are typically weak and may even vary with local time. However, energetic ions (above ~30 keV), which contribute significantly to the total ion pressure, show clear flux enhancements, leading to a rise in the total temperature from ~ 10 to ~ 16 keV. Preexisting perpendicular anisotropies in the thermal electrons are reduced during the substorm growth phase but become enhanced again after onset, sometimes after a brief period of parallel anisotropy. Similar anisotropy signatures are found for the thermal ions, although somewhat less pronounced.

Butterfly pitch angle distribution of relativistic electrons in the outer radiation belt: Evidence of nonadiabatic scattering

“…For example, at > 3∕2, we do not expect to observe any pitch angle diffusion for equatorial electrons [see Delcourt et al, 1995;Young et al, 2008], while at < 3∕2 this diffusion is important (see section 3 and Shibahara et al [2010]). Additionally, in the vicinity of the loss cone (at small pitch angles), the nonadiabatic scattering is nondiffusive (i.e., the averaged jump is not equal to zero and is always positive, see Delcourt et al [1994Delcourt et al [ , 1995), while analytical estimates give only Δ ∼ sin 0 with zero average value for a random uniform distribution of 0 [Howard, 1971;Cohen et al, 1978;Birmingham, 1984;Chirikov, 1987;Varma, 2003]. Moreover, as we discuss in Appendix B, most analytical expressions for Δ should be tested by numerical calculations because these expressions were derived using not well justified methods.…”

“…The ion fluxes do not exhibit a similar increase, thus, the spacecraft was likely located within the electron boundary of injection but eastward relative to the ion boundary [see Birn et al, 1997b, and references therein]. The injection is observed around the midnight sector where pure electron injections without observations of an ion flux increase are not rare (see statistics in Birn et al [1997a] Gkioulidou et al [2015].…”

“…In this Appendix B we give a brief overview of theories of the adiabatic invariant conservation and discuss in details the particular expression derived by Birmingham [1984]. Historically, the nonadiabatic scattering for particular plasma physical systems was investigated first for magnetic field configurations typical for laboratory mirror traps (see reviews [Baldwin, 1977;Chirikov, 1987]).…”

“…This simplification comes directly from our assumption about strong localization of a magnetic field perturbation, i.e., the scale is substantially smaller than the scale of dipole field inhomogeneity ∼ LR E . This model is widely used for investigations of charged particle scattering due to nonadiabatic motion [e.g., Gray and Lee, 1982;Birmingham, 1984;Delcourt et al, 1995]. The Hamiltonian of relativistic electrons with the rest mass m and the charge −e in this magnetic field has the following form:…”

“…To characterize the charged particle scattering in system (5), Birmingham [1984] derived the equation for the jump Δ Δ * ≈ C( * , ) cos 0 exp (…”