Space Plasma Physics The Study of Solar-System Plasmas (1978) / Chapter Skim
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Shock Systems in Collisionless Space Plasmas
Shock Systems in Collisionless Space Plasmas
Pages 54-125
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From page 54... ...
W Fredricks Space Sciences Staff TRW Defense 6 Space Systems Group 807
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From page 55... ...
And what if the gas is a plasma consisting almost entirely of ionized hydrogen, which is really two gases, one of protons, one of electrons? And what if the plasma is magnetized so that particle motion is related to the field and there is not one sound wave velocity but many types of waves with different velocities dependent on frequency and on direction in the plasma with respect to the magnetic field?
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From page 56... ...
The gas they describe is the solar wind that occupies the entire known solar system. Moreover, spacecraft instruments have disclosed a col 1isionless plasma shock in front of every one of the five planets visited so far, and still more of them are continually found traveling through the solar wind away from solar flares and ahead of certain plasma streams originated in the sun.
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From page 57... ...
The greatest advantage of spacecraft measurements Fn either the Earth's bow shock or interplanetary shocks is potential resolution of the fine structure by probes of dimensions much smaller than shock scale lengths. The offsetting disadvantage has been the restriction to single-point measurements of passing structures whose motion relative to the probe is usually unknown, so that ambiguous Doppler-shifted frequencies and scale lengths result.
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From page 58... ...
and TI parallel and perpendicular to the magnetic field B; their ratio Tp/Tj; the ratio of electron to ion temperature ' -L T /T.; the average density M = N » M ; the ratio of thermal to magnetic C 1 1*
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From page 59... ...
We then describe what is known of shock structures in space on both large and small scales by synopsizing the documented features of the Earth's bow shock system. The ensuing discussion touches on the properties of shocks elsewhere in the solar system and on the status of shock theory and then describes in some detail an example of how properties of the bow shock system are investigated with paired satellite measurements.
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From page 60... ...
There are two types of col 1isionless shocks of interest here. 0ne is a bow shock produced when an obstacle is placed in a "supersonic" plasma flow.
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From page 61... ...
Papers by Zhigulev ^ and Zhigulev and Romishevskii appear to be the first serious conjectures that the continuous solar wind plasma, upon encountering the intrinsic magnetic field of t;he earth, should produce a standing bow shock front. No dissipation or entropy-changing mechanism was specified in any detai1.
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From page 62... ...
The geophysica11y oriented theoretical work between about 1962 and 1968 proceeded largely along the lines of the "gas dynamic analog," in • which the col 1isionless MHD bow shock was studied grossly by reducing the MHD jump conditions for 8, v, p and pressure to a set of jump conditions analogous to those for shocks in gases. An excellent bibliography of such 98 efforts is contained in the review by Spreiter and Alksne .
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From page 63... ...
Paul. The 0G0-5 results, published by Fredricks et al.^ '" showed clearly that strong electrostatic waves were present in the larger magnetic field gradients within the structure of earth's bow shock.
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From page 64... ...
In spite of such drawbacks, these two results stimulated a significant theoretical thrust to investigate just what electrostatic instabilities could be produced by the current systems which produce the detailed magnetic field profiles in MHD col 1isionless shocks. This was further reinforced by the publication of a more comprehensive study of selected bow shock structures made by instruments aboard 0G0-5, and has been confirmed by instruments aboard subsequent flights, such as NASA's IMP-6,7,8 series, ESA's HE0S-1,2 and the USSR's Prognoz series.
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From page 65... ...
The nominal shock is not planar, but curved, so that its relationship to a uniform solar wind flow with velocity V-.,, carrying uniform field B... at an average 45° angle to ysw, is necessarily asymmetric.
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From page 66... ...
An enumeration of the qualifications to be attached to the very simplified Figure 1 serves as a virtual prescription for the next decade's study of shock structure in space. Before discussing these, the proton velocity distributions and the foreshock need to be mentioned.
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From page 67... ...
quasi-perpendicular bow shock is shown at upper right, while the sketch at upper left is Intended to represent the multimodal, undefined, but predominantly solar wind-like distribution In the quasi-para 1 lei structure, of which fragmentary cross sections have been obtained by the plasma detectors of several spacecraft. The question marks in Figure I accompany those characteristics of the depicted bow shock that represent reasonable extrapolations from measurement but are by no means established facts.
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From page 68... ...
Are return protons separable as a group? So far no plasma probe in the shock or magnetosheath has had the temporal or angular resolution to develop a reliable spectrum in velocity space.
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From page 69... ...
Mention of 8 and M provides the basis for the generalized qualification that the shock system exists in a multiparameter plasma in which every combination of values affects shock structure on either the macro or microscopic scale, or both. In addition, there are numerous physical properties of the plasma that simply do not appear in Figure 1 , all of which are of great interest: electron spectra, T /T., T\\+/Ti + , ct-particle contributions, |
From page 70... ...
No hint of such a dynamic situation is indicated in the static, two-dimensional, limited-parameter drawing offered here. General Scheme The gross effects of M and 8 on shock structure are summarized in a classification scheme devised by Dobrowolny and Formisano .
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From page 71... ...
Under q-parallel geometry, and for very high 8, strictly local combinations of parameters may play an important role in defining the processes taking place in local gradients. Microturfaulence An overall description of the wave properties that have been inferred from a combination of statistical and case-history studies in the bow shock is sketched in Figure 2.
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From page 72... ...
The role of wave-spectral form in shock characterization can be appreciated by viewing Figure 3. In the center, a magnetic profile along shock normal n is drawn for a typical supercritical, essentially perpendicular, bow shock in a warm solar wind, with electron and proton velocity space distributions superimposed.
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From page 73... ...
The electric wave noise at right is more complicated. Ahead of the main gradient, where there is a "foot" in the solar wind with associated small amplitude precursors, there is also a clear wave resonance at about 10I*
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From page 74... ...
In and behind the shock ramp, distributions represented by double, or even multiple bubbles, might be appropriate. Returning to Figure 2, the most important message of the figure is that microscale magnetic and electric wave activity are somewhat Independent of each other, but are both low for the bow shock in cold, low M solar wind flow and both high for the bow shock in hot, high M solar wind flow.
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From page 75... ...
await the outcome of future investigations, e p' p| pj_ Foreshock One of the most interesting discoveries of space plasma physics has been the shock-generated region of particles, waves, and wave-particle interactions that inhabit a vast volume of space outside the bow shock. The existence of such a region where B is parallel to n was broadly predicted *
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From page 76... ...
The result is a modified spiral trajectory displaced from B-w in the shock frame, with net guiding center velocity V » Vn + V,, where V • Vn » 0, as shown in Figure k (a)
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From page 77... ...
signify the region occupied by return electrons. Most return protons are appreciably slower than electrons (p « 2)
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From page 78... ...
The varieties of particle energies and wave frequencies that have been detected outside the bow shock in the cislunar region have been enumerated kk In a recent summary review . Here, we shall review newer results and describe the current outstanding issues involving the foreshock.
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From page 79... ...
set out to examine the thermal properties of the protons in the proton foreshock and discovered no temperature effect but a slightly lower thermal anisotropy. The data used for the study, however, did not include a magnetometer or return proton detector, so the direction of the electron heat flux was used as a guide to the ambient regime applicable to each data point.
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From page 80... ...
6 28 Auer et a_l. and Fbrmisano and Amata undertook statistical analyses of the solar wind proton properties associated with the ULF foreshock and reported what appeared to be distinguishable differences between N, V, and B2/N2 in the foreshock and in the unperturbed solar wind.
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From page 81... ...
Magnetosheath The entire region between a detached bow shock and the obstacle It shields from a surrounding supersonic flow is not ordinarily thought of when referring to the post shock fluid. In the case of a magnetospheric
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From page 82... ...
For example, the magnetic ramp, or gradient, I.e., the "jump," is not strictly superposed on the density ramp. Especially in the cases of extreme parameter values such as very high betas or Mach numbers, a delayed or expanded particle redistribution mechanism may require appreciable post-shock data collection to deduce the physical processes
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From page 83... ...
Col 1isionless shocks in the solar wind are three-dimensional, with magnetic field lines connecting one region of the nominal surface, through the post shock standoff region, to the other "side." If one side Is q-perpendicular in a uniform field, the other side is q-parallel. Hence there is the possibility of communication by field-aligned phenomena propagating from one part of the nonuniform shock to another through the downstream region.
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From page 84... ...
Deeper in the earth's magnetosheath, the most pertinent results are, first, that ion acoustic wave noise continues to modify the solar wind electric wave spectrum well behind the quasi-perpen81 dicular magnetic ramp , and, second, that suprathermal protons of energy > 100 keV appear in sharp correlation with large amplitude magnetic field 102 oscillations similar to those of q-parallel pulsations . Improvements in instrumentation and expanded data analysis will be necessary to catalogue magnetosheath properties and relate them to the bow shock.
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From page 85... ...
The local characteristics of shocks of both categories, with regard to magnetic field and electron behavior, are familiar from the earth's bow shock.
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From page 86... ...
One is the moon, which having no intrinsic field of dimension comparable to itself, but possessing small local conductive or magnetic anomalies, offers the opportunity to examine marginal col 1isionless 91 shock formation and maintenance near the scale limits required by theory A second case is Venus, where the obstacle in the solar wind's path is neither a strong magnetosphere nor the absorptive surface of the planet itself (or its atmosphere) , but an induced magnetic field arising from currents in the ionosphere.
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From page 87... ...
There is only one example of detailed study of an alien bow shock system from a plasma physical viewpoint. Fairfield and Behannon analyzed magnetic field data from Mariner 10 and found whistler waves propagating upstream from the shock as observed previously at the earth .
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From page 88... ...
Thus, multiple charged particle acceleration associated with a shock front could result in particles developing cosmic ray energies before being released by local alterations in field or shock geometry to escape into the general interstellar environment. The speed of a driven shock will depend on the speed of the plasma piston that drives it.
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From page 89... ...
If such examples are multiplied, there are numberless opportunities for particles to be accelerated throughout the universe by shock waves of many origins. Study of the energization mechanism taking place in the solar system should have direct application to such astrophysical sources.
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From page 90... ...
The subject has been discussed separately for elec23 59 trons and for protons . More to the point, there has been both analysis and observation of high energy ions in connection with interplanetary shocks, 4 86 87 and this subject is currently under investigation °'0' .
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From page 91... ...
. Furthermore, observations of 0GO-5 data on bow shock crossings revealed shock structures in which electrostatic turbulence appeared in many magnetic gradients, without randomization of the proton flow.
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From page 92... ...
of bow shock structures, so that exact identification of the microinstabi1ity responsible for the dissipative process in each case is not an easy task. It should be pointed out that measurements of lower hybrid resonance turbulence in the earth's bow shock structures have not been made in the past due to lack of Instrumentation to cover the extremely low frequency electrostatic spectrum (f.u_ - 10-20 Hz)
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From page 93... ...
of the proton velocity distribution behind bow shocks, without introducing subshocks or wave instabilities as ion heating mechanisms. This idea is reminiscent of the ion "orbit-crossing" mechanism for ion randomization discussed by Auer et aj.
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From page 94... ...
The long dashed curve shows the magnetosonic Mach number that would apply to a planetary, or cometary, shock stationary in a 400 Km/sec solar wind at any r, but the circled points mark only the M-values at the permanent planets, identified by their initials just above the horizontal coordinate line. The Q-turbulent/turbulent designation under the line denotes the category of planetary shock structure defined by the M _ and B combinations in the corresponding regions.
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From page 95... ...
At Mars and Jupiter, however, and by inference, beyond them, the foreshocks do not reach sunward of the subsolar points of their bow shocks, since Q^ < 90°, but they may occupy a considerable region to the sides of the shock flanks. The trend in 9 _ also implies that, given the usual fluctuations of the IMF direction, there Is a fair probability that the foreshocks of the inner planets will occupy their entire upstream regions a substantial fraction of the time, i.e., when 9..- slews toward 0°.
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From page 96... ...
According to MS B these, the bow shock system of the earth alone can occur in any combination of parameters, thus providing a rich table from which to select samples for the study of shock structure, both in transient and steady-state conditions. In reality, not all combinations of the structural parameters are equally probable or equally observed.
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From page 97... ...
Both are chosen as monitors of local ion acoustic noise, which predominates in and around the bow shock. Descriptions of the various instruments can be found in references ^*
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From page 98... ...
The B wavy line F in the same insert is the forwardmost proton-foreshock boundary that applied during this early interval. IMP-8 was obviously in the foreshock then, and we see that electric wave noise was enhanced (top panel)
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From page 99... ...
In this case the field projection was actually opposite the small arrows, which have been drawn for visual clarity and to emphasize the direction of possible particle velocities toward the satellite. It cannot now be stated with certainty whether the enlarged E-fleld amplitudes at IMP-8 were related to the bow shock or to changed interplanetary conditions, but we observe that the wavy foreshock boundary refers only to the region containing slow protons with UH s 2 V_w and their associated ULF upstream waves.
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From page 100... ...
It remains only to note the short entry and exit of the bow shock itself by "lMP-7 just before 2300, as determined from the thermalization and deflection of protons and the sharp peak in plasma wave noise. We proceed to July 5th, in Figure 8.
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From page 101... ...
The satellite reentered the bow shock at 2210 under conditions varying between quasi-perpendicular and quasi-parallei and remained in the magnetosheath through the end of the 5th. The IMP-8 data of 5 July featured electric wave activity which was low, with some sporadic enhancement through about 1630, and then increased for the remainder of the day.
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From page 102... ...
The four plasma panels, for example, were selected from a still wider display of the solar wind's properties developed at Iowa to show the properties of the multidimensional solar wind in metric and velocity space in a manner rapidly understandable to the data analyst. The verbal description of the events of k-5 July, how
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From page 103... ...
In addition to conventional field and particle devices, high resolution, omnidirectional plasma detectors will be of great importance. The earth's bow shock will be the principal source of new, detailed measurement covering almost the whole range of solar wind parameter combinations.
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From page 104... ...
Figures 7 and 8 of the foregoing section illustrate, scandalously, the first published example of double measurements in the foreshock with the instrumentation shown. A commitment is urgently needed to recapture the investment in plasma physics lying idly in data warehouses throughout the nation.
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From page 105... ...
858 in ^^ • PO tTi *
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From page 107... ...
CO D O io -- 4V 4V -- 4i IB 00 z Z X a.
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From page 108... ...
Z Z x tfl 4> U l C C 4-> I I i u- in tfl C -- o o »•^ M 4)
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From page 110... ...
100 M * Q - PERPENDICULAR ® PERPENDICULAR A Q - PARALLEL FIGURE 2 Conceptualization of bow shock microphenomenology as represented by electromagnetic noise power density and plasma electric wave noise amplitude in the various shock structures determined by upstream plasma parameters 0 and M
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From page 111... ...
Electron and proton distributions in velocity space are shown in the cold, fast solar wind, foreground, in the foot of the magnetic structure where the electrons are initially heated and the protons retarded and partially scattered, in the center of the principal magnetic gradient, or ramp (shaded) where the electrons are fully scattered and the protons partially heated and scattered to form a bimodal distribution, and finally behind the magnetic front, where electrons and protons are both found heated and scattered into nonmaxwellian distributions.
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From page 112... ...
Average field longitude 0B and foreshock boundary angle 0X F vs heliocentric distance.
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From page 113... ...
Positions of the satellites relative to the bow shock system are shown in the diagrams superposed on the second panel from the top.
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From page 115... ...
A., Energetic electrons of terrestrial origin behind the bow shock and upstream in the solar wind, J Geophys.
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From page 116... ...
H., Bow shock associated waves observed In the far upstream interplanetary medium, J Geophys.
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From page 117... ...
26. Formisano, V., Jupiter's bow shock structure, Lett.
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From page 118... ...
T Russell, 0GO 5 observations of electrostatic turbulence in bow shock magnetic structures, J
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From page 119... ...
W., Binary index for assessing local bow shock obiiquity, J Geophys.
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From page 120... ...
E Holzer, Structure of a quasi-parallei, quasilaminar bow shock, J
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From page 121... ...
W Greenstadt, Thickness of magnetic structures associated with the earth's bow shock, J
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From page 122... ...
J Smith, High-frequency magnetic fluctuations associated with the earth's bow shock, J
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From page 123... ...
A Gurnett, Correlation of bow shock plasma wave turbulence with solar wind parameters, J
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From page 124... ...
and B Goldstein, Solar wind interaction with lunar magnetic fields, J
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From page 125... ...
A., The earth's bow shock wave, J Geophys.
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