Acceleration of electrons by interplanetary shocks

Abstract

Although proton acceleration often is observed at interplanetary shocks, electron acceleration rarely has been reported. In contrast, many of the shocks identified so far by instruments on the ISEE 3 spacecraft show significant increases in the spin averaged electron flux at energies greater than 2 keV. At 2 keV, the spin averaged electron flux usually increased by a factor of 2 to 3 but on several occasions increased to more than 10 times its preshock level. Often, the behavior of the electrons is similar to higher energy (several MeV) proton spikes: the electrons are field aligned before the shock and are highly anisotropic with the flux maximum perpendicular to the magnetic field at the shock and immediately after the shock. This is the signature of the shock drift model of acceleration where the gradient in the magnetic field causes the particles to move in the direction of lower potential energy in the convection electric field. Given that a primary assumption of the shock drift model is that the gyroradius is much larger than the shock thickness, and that this assumption is invalid for electrons, it is initially surprising that electrobs and ions can act similarly at shocks. A closer look shows that by conserving the first adiabatic invariant, electrons obey the same expressions for reflected and transmitted particles as ions.of the ratio of thermal to magnetic energy fl(---1). The mach number for the bow shock is in the range 6-10 whereas interplanetary shocks are generally lower at 2-3. Ion acceleration is now a well known phenomenonat both interplanetary shocks [Bryant eta!., 1962; Armstrong eta!., 1977] and the earth's bow shock [Asbridge eta!., 1968; Lin eta!., 1974; Paschmann eta!., 1980]. Less frequently observed is electron acceleration. Some early reports at the bow shock are by Anderson [1968, 1969]. More recently, Anderson et al. [1979a] report that electrons are accelerated in thin sheets along field lines tangent to the shock. The only reports of electron acceleration at interplanetary shocks seem to be Armstrong and Krimigis [1976] on October 29, 1972, for energies >200 keV, McGuire [1976] for December 16 and 17, 1971, and May 15, 1972, for energies as low as 0.5 keV, and three unpublished events referred to by Decker [1981]. This paper reports that electron acceleration occurs at many shocks. INSTRUMENTATION The ISEE 3 (International Sun Earth Explorer) spacecraft is ideally suited for making measurements of interplanetary particles without interference from the Earth. Launched August 12, 1978, its unique orbit about the libration point on the earth-sun line (Ll, 230 Re from the earth) places it continuously in the interplanetary medium. The University of California energetic particles experiment [Anderson eta!., 1978; Potter, 1981] was designed to measure electrons from 2 to 1000 keV and ions from 50 keV to 40 MeV with high sensitivity. Briefly, the electrostatic analyzer consists of---10-cm diameter hemisphere

Shock Drift Acceleration of Electrons

“…Armstrong & Krimigis 1976;Potter 1981;Sarris & Krimigis 1985;Lopate 1989). A characteristic feature of such accelerated electrons is a beam-like structure, which involves localisation in velocity space not only in the beam direction, but also in beam speed, with a gap between the speeds of thermal electrons and the beam particles.…”

Drift Acceleration at Interplanetary Shocks

“…Bidirectional flow (but without loss cones) of protons as well as electrons were also observed by Potter (1981) upstream of quasi-perpendicular shocks. Figure 8 shows the angular distribution of 2 keV electrons just prior to the arrival of a shock on 1979 July 26.…”

Polarization and Fragmentation of Solar Type II Radio Bursts

“…This mechanism involves (1) energization of electrons by the outwardpropagating coronal or IP shock, either due to reflection at the shock into upstream regions or due to the leakage from the heated downstream into upstream regions, (2) excitation of Langmuir waves through electron streaming instabilities, and (3) conversion of Langmuir waves into escaping radiation at f pe and 2f pe . In addition, the energetic electron flux enhancements as well as the Langmuir wave occurrences do not show any relationship with either the shock normal angle B n or the fast magnetosonic Mach number M ms (Potter 1981;Tsurutani & Lin 1985;Thejappa & MacDowall 2000). For example, Urbarz et al (1977) have suggested that the fluctuations in the type II burst fluxes are connected to the magnetic field and electron density fluctuations in the corona.…”

“…Some of these observations show that (1) neither energetic electron (Potter 1981;Tsurutani & Lin 1985) nor Langmuir wave occurrence (Thejappa, MacDowall, & Vinas 1997;Thejappa & MacDowall 2000) is correlated with shock parameters, such as the shock normal angle B n and the fast magnetosonic Mach number M ms , (2) the site of type II burst excitation is probably located in the upstream regions (Lengyel-Frey et al 1997;Bale et al 1999;Thejappa & MacDowall 2000;Reiner et al 2000), and (3) the fragmentation of the type II radio bursts can be due to fluctuations in the efficiency of the conversion of Langmuir waves into escaping radiation (Thejappa & MacDowall 2001). Thejappa & MacDowall (2001) have suggested that fluctuations in the efficiency of linear coupling of Langmuir waves with the escaping radiation are probably responsible for the fragmentation.…”