Engineering topics

Khurana, K. K.

Publications and source records attributed to Khurana, K. K..

25 records · Page 2

NASA NTRS1995

Solar wind interaction with small bodies. 2: What can Galileo's detection of magnetic rotations tell us about Gaspra and Ida

As the Galileo spacecraft passed the asteroids Gaspra in 1990 and Ida in 1993, the magnetometer recorded changes in the solar wind magnetic field that we associate with the presence of the nearby body. This paper focuses on the types of interactions that can produce perturbations in the solar wind. We have suggested that the interaction at Gaspra is consistent with expectations of flow diversion by a magnetic dipole moment and an associated 'magnetosphere' whose scale size is much larger than the diameter of the solid body. The conditions for the Ida flyby leave more room for ambiguity. The observations could plausibly be related to either interaction with a magnetized body or with a conducting body. We will report on details of the observations that may enable us to distinguish between the different types of interaction and to provide quantitative estimates of the physical properties of the asteroids themselves.

Kivelson, M. G.↗

NASA NTRS1995

Galileo flybys of Earth: The nature of the distant shock

We report on observations of the Earth's bow shock at unprecedentedly large downtail distances, some as remote as 360 R(sub E). Suprisingly, we find that even at these large distances, the bow shock signature remains clear. The cases we report are among the weakest shocks ever clearly identified. These shocks reveal patterns of field changes remarkably similar to those observed for stronger shocks. Indeed, several of the shocks could serve as textbook examples because they occur in unusually quiet and steady solar wind conditions. The quasi- perpendicular shocks are, in some cases preceded by whistler wave trains. Several of the shocks which have normal vectors in the transitional region between quasiperpendicular and quasiparallel are associated with large amplitude wave disturbances in the downstream plasma. Although low Mach number, these shocks are the source of large amplitude turbulence. We show a range of fits to the locations of the shock crossings, and argue that, for steady solar wind conditions, there is no sign of tail flapping. The displacement of the shock from the tail axis appears to be governed by the interplanetary magnetic field orientation as previously reported for Venus.

Kivelson, M. G.↗

NASA NTRS1993

ULF waves in other magnetospheres - observations and possible source mechanisms

Five other planets besides the Earth (Mercury, Jupiter, Saturn, Uranus and Neptune) in our solar system are now known to possess internal magnetic fields. The exploration of these planets by the Mariner, Pioneer, Voyager and Ulysses spacecraft has revealed that all of them possess fully expressed magnetospheres which share several similarities in their structures with the Earth's magnetosphere. This paper presents an overview of the work done so far in the field of the ULF waves in the magnetospheres of Mercury, Jupiter, Saturn and Uranus. To give an idea of the expected wave periods, gyroperiods of the dominant ion species and the fundamental periods of the standing Alfven waves are presented as functions of L parameter in these magnetospheres. In the magnetosphere of Mercury, ULF waves were observed in the vicinity of the magnetopause and in the inner magnetosphere with frequencies in the range of 0.1-0.5 Hz. In the magnetosphere of Jupiter, at least three different types of wave sources are observed. Near the dayside and the dawn magnetopause, waves with periods 5-20 min and amplitudes between 5 and 10 nT are observed which may be caused by an interaction between the corotating outflowing plasma and the antisunward moving plasma from the magnetosheath. In Saturn's magnetosphere, ULF waves have been observed to be strongly confined to the plasma sheet and have wave periods in the range of 5-60 minutes. The calculated fundamental has a wave period of 5-6 hours in the region where these waves were observed. The ULF waves have extremely small amplitudes (approximately = 0.3 nT) in the magnetosphere of Uranus. These waves were also seen to be confined to the low magnitude latitudes and have periods much shorter than that of the fundamental of a standing Alfven wave.

Khurana, K. K.↗

NASA NTRS1993

Magnetic field signatures near Galileo's closest approach to Gaspra

Two large magnetic field rotations were recorded by the spacecraft Galileo 1 minute before and 2 minutes after its closest approach to the asteroid Gaspra. The timing and the geometry of the field changes suggest a connection with Gaspra, and the events can be interpreted as the result of the draping of the solar wind field around a magnetospheric obstacle. Gaspra's surface field is inferred to be within an order of magnitude of Earth's surface field, and its magnetic moment per unit mass is in the range observed for iron meteorites and highly magnetized chondrites. The location of the magnetic signatures suggests that perturbations are carried by waves in the magnetosonic-whistler mode with wavelengths between electron and ion gyro radii.

Kivelson, M. G.↗

NASA NTRS1993

The Galileo Earth encounter - Magnetometer and allied measurements

An overview of the Galileo magnetometer observations from the crossing of the tail magnetopause at an antisolar distance of close to 100 R(E) through exit into the solar wind on the dayside is presented. These measurements are linked with correlative data from ground stations and from IMP 8 which was ideally located to serve as a monitor of the solar wind upstream of the bow shock. A time line of the important geomagnetic events of the day that provides a framework for the full multiinstrument analysis of the flyby data is presented. The observations are used to investigate apsects of the relationship between magnetotail dynamics and the separate intensifications of a multiple onset substorm inferred from ground-based data. It is proposed that the signatures associated with individual substorm intensifications are localized in the dawn-to-dusk extent even at remote locations in the magnetotail, just as they are in the ionosphere, and that the tail disturbances associated with successive substorm intensifications step across the tail towards the dusk flank.

Kivelson, M. G.↗

NASA NTRS1991

Magnetic field studies of the solar wind interaction with Venus from the Galileo flyby

During the February 10, 1990 flyby of Venus, the Galileo spacecraft skimmed the downnstream flank of the planetary bow shock. This provided an opportunity to examine both the global and the local structure of the shock in an interval during which conditions in the solar wind plasma were quite steady. The data show that the cross section of the shock in planes transverse to the flow is smaller in directions aligned with the projection of the interplanetary magnetic field than in directions not so aligned. Ultralow-frequency waves were present in the unshocked solar wind, and their amplitude peaked when the spacecraft was downstream of the foreshock. At large distances down the tail, the Mach number of the flow normal to the shock is low, thus providing the opportunity to study repeated crossings of the collisionless shock in an interesting parameter regime. Some of the shock crossings reveal structure that comes close to the theoretically predicted form of intermediate shocks, whose existence in collisionless plasmas has not been confirmed.

Kivelson, M. G.↗

NASA NTRS1987

Voids in Jovian magnetosphere revisited - Evidence of spacecraft charging

The Voyager 2 Plasma Science Instrument (PLS) measuring cold plasma number density observed about a dozen 'voids', lasting from a few minutes to 20 min, in the vicinity of the Ganymede-orbit crossing, when the low-energy ion and electron fluxes recorded fell to very low levels. Original interpretations associated these 'voids' with Ganymede wake effects. In the present study, the PLS data are reexamined, in conjunction with data from the magnetic field experiment and the low-energy charged particle (LECP) experiment. The LECP data showed that the PLS voids were accompanied by large enhancements of the flux of energetic electrons and ions, while the magnetic data exhibited no systematic signatures. It is suggested that increased energetic electron fluxes in the void regions intermittently charged the spacecraft negatively to values between a few kV and a few tens of kV, and that spacecraft charging could have produce dropouts in the measured cold ion and electron fluxes and enhancements in the measured fluxes of hot particles consistent with the observations.

Khurana, K. K.↗