C. B. Wang

Influence of the solar wind dynamic pressure on the decay and injection of the ring current

The influence of the solar wind dynamic pressure on the decay and injection of thering current is investigated empirically, on the basis of the solar wind and the geomagneticindex Dst of the OMNI database, for the period from January 1964 to July 2001. Wefound that when the position of the ring current is closer to the Earth for a higher solarwind dynamic pressure, the decay time of the ring current decreases. The decay time, inhours, varies as follows, t = 8.70 exp(6.66/(6.04 + P)), for northward interplanetarymagnetic fields (IMF), where P is the solar wind dynamic pressure in nanopascals. It isalso found, by minimizing the root mean square errors of the hourly Dst differencebetween the calculated values and the measured ones, that the ring current injection rate isproportional to the solar wind dynamic pressure, with a power index equal to 0.2 duringsouthward IMF. This implies that the ring current injection increases when themagnetosphere is more compressed by high solar wind dynamic pressure. On the basis ofour new results we demonstrate that the predictions of Dst using O’Brien andMcPherron’s [2000a] model are improved, especially for intense geomagnetic stormswhen the influence of the solar wind dynamic pressure on the decay and injection of ringcurrent is taken into consideration.

Intense duskside lower band chorus waves observed by Van Allen Probes: Generation and potential acceleration effect on radiation belt electrons

Local acceleration driven by whistler mode chorus waves largely accounts for the enhancement of radiation belt relativistic electron fluxes, whose favored region is usually considered to be the plasmatrough with magnetic local time approximately from midnight through dawn to noon. On 2 October 2013, the Van Allen Probes recorded a rarely reported event of intense duskside lower band chorus waves (with power spectral density up to 10(-3)nT(2)/Hz) in the low-latitude region outside of L=5. Such chorus waves are found to be generated by the substorm-injected anisotropic suprathermal electrons and have a potentially strong acceleration effect on the radiation belt energetic electrons. This event study demonstrates the possibility of broader spatial regions with effective electron acceleration by chorus waves than previously expected. For such intense duskside chorus waves, the occurrence probability, the preferential excitation conditions, the time duration, and the accurate contribution to the long-term evolution of radiation belt electron fluxes may need further investigations in future.

GENERATION OF TYPE III SOLAR RADIO BURSTS IN THE LOW CORONA BY DIRECT AMPLIFICATION

An alternative scenario to the plasma-emission model is proposed for coronal type III solar radio bursts. According to this model, the radio bursts are produced inside a magnetic flux tube with density depletion by a direct amplification of electromagnetic waves with frequencies near the electron gyrofrequency and its harmonics. The amplification mechanism is the cyclotron-maser instability driven by a beam of flare-generated streaming electrons. In the present discussion, a depletion factor of approximately 102 near the chromosphere is assumed. The essential point is that in order to produce the electromagnetic waves near the fundamental electron gyrofrequency, the present model requires 0.1 ≤ fp/fg ≤ 0.4 (where fp and fg denote the plasma frequency and gyrofrequency, respectively) in the source region. The propagation of an amplified wave is initially confined within the magnetic flux tube until the wave arrives at a point where the local exterior cutoff frequency is equal to the exiting wave frequency. The proposed model is spurred by the consideration that above an active region where the emission is presumed to originate, the ambient magnetic field is strong enough that, in contrast to conventional theories, it cannot be ignored. Preliminary analysis leads to some encouraging results, on the basis of which we may resolve a number of long-standing issues raised by observations. The proposed scenario also implies a fundamentally different interpretation of the observed frequency drift in the dynamic spectrum.

Pseudoheating of protons in the presence of Alfvénic turbulence

The possibility of heating of protons via Alfven waves is a topic that stimulates much discussion in both plasma physics and astrophysics. Conventional thinking is that dissipation is essential for heating. In two recent discussions it is shown that turbulent Alfven waves can enhance stochastic particle motion via scattering. This process can lead to a higher proton temperature. In this Letter two essential points are stressed: First, there is no dissipation; second, physically the temperature increment is “apparent” rather than genuine, and consequently the heating is spurious. If the turbulent wave field should diminish, the proton temperature returns to its original value. The purpose of this communication is to elucidate and explain the above points.

Resonant wave-particle interactions modified by intrinsic Alfvenic turbulence

The concept of wave-particle interactions via resonance is well discussed in plasma physics. This paper shows that intrinsic Alfven waves can qualitatively modify the physics discussed in conventional linear plasma kinetic theories. It turns out that preexisting Alfven waves can affect particle motion along the ambient magnetic field and, moreover, the ensuing force field is periodic in time. As a result, the meaning of the usual Landau and cyclotron resonance conditions becomes questionable. It turns out that this effect leads us to find a new electromagnetic instability. In such a process intrinsic Alfven waves not only modify the unperturbed distribution function but also result in a different type of cyclotron resonance which is affected by the level of turbulence. This instability might enable us to better our understanding of the observed radio emission processes in the solar atmosphere.

Generation of Type III Solar Radio Bursts in the Low Corona by Direct Amplification. II. Further Numerical Study

In a recent paper a cyclotron maser model was proposed as an alternative explanation of coronal type III solar radio bursts. However, the discussion was preliminary in that it was limited to a local theory and in that the propagation of the emitted waves was studied with a simplified scheme. The present article complements the earlier study by simultaneously computing the ray trajectory with a more sophisticated approach and estimating the intensity of the amplified waves. Numerical results lead to the conclusion that the O1-mode is insignificant despite the robust initial growth rate predicted by local theory, while X1- and X2-modes are equally important. This finding is consistent with observations and confirms the scenario originally put forward in the earlier paper.

Pitch-angle diffusion of ions via nonresonant interaction with Alfvénic turbulence

The present discussion revisits the problem of nonresonant heating of ions by Alfvenic turbulence. It is shown that in the limit of weak Alfvenic turbulence it is appropriate to describe the nonresonant heating of protons as perpendicular pseudoheating. However, in a more general situation it is demonstrated that the more appropriate view of the nonresonant heating process is the pitch-angle scattering in the wave frame. The purpose of this paper is to generalize the earlier theory to the case in which the energy density of the turbulent Alfven waves is not necessarily very low. For weakly turbulent situation the present analysis confirms the earlier finding by Wu and Yoon [Phys. Rev. Lett. 99, 075001 (2007)], according to whom the nonresonant Alfven wave heating is described as leading to perpendicular pseudoheating of the protons. However, for more general situation the present paper demonstrates that pitch-angle scattering plays the principal role in the Alfven wave pseudoheating process, and thereby s...

Ring-beam driven maser instability for quasiperpendicular shocks

The cyclotron maser instability is a well-known radiation emission mechanism responsible for radio emissions in magnetized planets and for laboratory microwave generation devices. The present paper discusses mechanisms and properties of cyclotron maser instability driven by a ring-beam distribution of energetic electrons with application to the quasiperpendicular collisionless shock. It is shown that the fast extraordinary and ordinary electromagnetic waves as well as slow upper-hybrid Z-mode are excited over a wide range of physical parameters. The implications of the present findings for actual applications including the coronal type II radio source are also discussed.

ALTITUDE-DEPENDENT EMISSION OF TYPE III SOLAR RADIO BURSTS

The present discussion complements a preceding article in which a cyclotron-maser theory of type III solar radio bursts is proposed. One important issue, which has not been addressed in any of the existing theories, is that in the event of a F-H pair emission dynamic spectra usually show an initial time delay of the fundamental (F) component after the harmonic (H) component has commenced. Moreover, the ratio of the starting frequencies of the H waves to those of the F waves is generally higher than 2. A plausible interpretation is that the emission of H waves starts at an altitude lower than that for F waves. This notion leads to the present study. Although it is formulated within the context of the cyclotron-maser scenario, the model of the source electrons is different from that discussed previously.

Anomalous dynamics of the extremely compressed magnetosphere during 21 January 2005 magnetic storm

Dynamics of the dayside magnetosphere and proton radiation belt was analyzed during unusual magnetic storm on 21 January 2005. We have found that during the storm from 1712 to 2400 UT, the subsolar magnetopause was continuously located inside geosynchronous orbit due to strong compression. The compression was found to be extremely strong from 1846 to 2035 UT when the dense plasma of fast erupting filament produced the solar wind dynamic pressure Pd peaked up to >100 nPa and, in the first time, the upstream solar wind was observed at geosynchronous orbit during almost 2 hours. Under the extreme compression, the outer magnetosphere at L > 5 was pushed inward and the outer radiation belt particles with energies of several tens of keV moved earthward, became adiabatically accelerated and accumulated in the inner magnetosphere at L 20%, which is well appropriate for erupting filaments and which is in agreement with the upper 27% threshold for the He/H ratio obtained from Cluster measurements.