Results are presented from a preliminary analysis of data obtained near Mercury on 29 March 1974 by the NASA-GSFC magnetic field experiment on Mariner 10. Rather unexpectedly, a very well-developed, detached bow shock wave, which develops as the super-Alfvenic solar wind interacts with the planet, has been observed. In addition, a magnetosphere-like region, with maximum field strength of 98 gammas at closest approach (704 kilometers altitude), has been observed, contained within boundaries similar to the terrestrial magnetopause. The obstacle deflecting the solar wind flow is global in size, but the origin of the enhanced magnetic field has not yet been uniquely established. The field may be intrinsic to the planet and distorted by interaction with the solar wind. It may also be associated with a complex induction process whereby the planetary interior-atmosphere-ionosphere interacts with the solar wind flow to generate the observed field by a dynamo action. The complete body of data favors the preliminary conclusion that Mercury has an intrinsic magnetic field. If this is correct, it represents a major scientific discovery in planetary magnetism and will have considerable impact on studies of the origin of the solar system.

https://science.sciencemag.org/content/sci/185/4146/151.full.pdf

Magnetic Field Observations near Mercury: Preliminary Results from Mariner 10.

Discontinuities in an anisotropic plasma and their identification in the solar wind

Power spectra and fluctuations of interplanetary magnetic field from Mariner 4 data for solar active and quiet days

Power spectra and discontinuities of the interplanetary magnetic field - Mariner 4.

ISEE 1, 2, 3, IMP 8, and Prognoz 7 observations of interplanetary shocks in 1978 and 1979 provide five instances where a single shock is observed by four spacecraft. These observations are used to determine best-fit normals for these five shocks. In addition to providing well-documented shocks for future investigations these data allow the evaluation of the accuracy of several shock normal determination techniques. When the angle between upstream and downstream magnetic field is greater than 20 deg, magnetic coplanarity can be an accurate single spacecraft method. However, no technique based solely on the magnetic measurements at one or multiple sites was universally accurate. Thus, the use of overdetermined shock normal solutions, utilizing plasma measurements, separation vectors, and time delays together with magnetic constraints, is recommended whenever possible.

Multiple spacecraft observations of interplanetary shocks Four spacecraft determination of shock normals

We present initial results from a survey of the effects of interplanetary shocks on energetic ≥2 keV electrons and ≥47 keV ions, as observed by the field, plasma, and energetic particle experiments on the ISEE 3 spacecraft. Shock normals, velocities, Mach numbers, and upstream and downstream plasma parameters were determined for 37 forward shocks out of a total of 55 shocks observed between August 1978 and November 1979. We find that a minimum shock velocity along the upstream magnetic field of ∼250 km/s is required for an interplanetary shock to have a significant effect on acceleration of ≥2 keV electrons or ≥47 keV ions. Shocks with no effect on the energetic particle populations also had relatively small ratios of downstream to upstream magnetic field magnitudes. These results suggest that magnetostatic reflection off the shock itself is a significant mechanism in the acceleration process. Both energetic electron and ion flux variations associated with shocks can be classified into four general types: (1) no significant variation at all, (2) a spike of a few minutes duration at or near the shock, (3) a steplike postshock increase, and (4) a slow rise beginning several hours before the shock (energetic storm particle event). Essentially, every quasi-perpendicular shock with θBn ≳ 70° produced a shock spike in the proton fluxes, while every quasi-parallel shock (θBn ≲ 50°) produced a proton energetic storm particles event, provided the shock velocity was greater than the above stated threshold. Electron spikes were also observed for most, but not all, shocks with θBn ≳ 70°. The most common effect observed in the electron fluxes was a steplike postshock increase of a factor of ∼2. These had no obvious dependence on θBn, but were found for every shock with speed greater than ∼175 km/s. Shock effects in the electron fluxes were about as common as for protons, but were limited to the 2–10 keV energy range except for 3 events which extended up to ∼50 keV. We find that significant ambient populations of both ≥2 keV electrons and ≳47 keV ions are present in the interplanetary medium prior to every shock. These particles could be the “seed” particles for the shock acceleration.

Acceleration of >47 keV Ions and >2 keV electrons by interplanetary shocks at 1 AU

The nonuniform emission of the solar wind from the sun means that conditions are established which favor the development of discontinuities in the plasma parameters. Since the solar wind is in rapid proper motion with respect to the sun and the earth, examination of these discontinuities requires that the wind velocity be transformed away. Then it is found that they satisfy the conditions of magnetohydrodynamics and can be treated as shock waves and the stationary contact surfaces consisting of either tangential or contact discontinuities. The collision-free structure of the solar wind suggests that the tangential discontinuity is the more likely contact surface as it is more capable of inhibiting diffusion which is required for a lifetime sufficient for the structure to be carried to the neighborhood of the earth. Either the shock wave or the contact surface can create signals that are detectable at the surface of the earth. The simplest surface signal to detect is the sudden impulse (SI) but other signals may be found. The existence of a field of MHD discontinuities in the solar wind should make possible the generation of ensembles of shocks and contact surfaces. Various possibilities are explored and these are discussed from the standpoint of combinations of sudden impulses at the earth's surface which are both positive and negative. Some of these are recurrent with a 27-day period; the interplanetary M region shock ensemble associated with this is discussed and the development of these structures in space is reviewed. Lastly observational evidence for interplanetary shock waves is given together with the analytic technique for establishing their geometry and comparing the derived and measured jump parameters. The applicability of the geometrical construction of the general class of MHD discontinuity to their analysis is indicated and shows the way in which the structural content of the solar wind can be classified by the use of magnetometers and plasma probes. A parametric study of the jump conditions through a shock wave can be used to verify the correctness of field measurements because of the redundancy in measurements. This also allows the details of shock structure to be examined including the intrinsic partitioning of the internal energy of the shocked plasma.

D. S. Colburn

Papers

Discontinuities in the solar wind