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Patel, V. L.

Publications and source records attributed to Patel, V. L..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Low-frequency electromagnetic waves driven by gyrotropic gyrating ion beams

The origin of left- and right-hand-polarized low-frequency waves in space plasmas is analyzed. It has been shown that a gyrotropic gyrating ion beam, a ring in velocity space, can excite electromagnetic modes in the plasma near the beam gyrofrequency. It excites left-hand-polarized shear Alfven waves and their harmonics via the coupling of Alfven modes with the beam modes. It can also excite right-hand-polarized fast-mode magnetosonic waves and their harmonics as well. The excitation is possible for beam ions heavier than the plasma ions. The growth rate varies as one-third power of the beam density and decreases with the angle of wave propagation with respect to the ambient magnetic field. The nonlocality has a stabilizing effect on the instability. The predicted values of the wave frequencies compare reasonably well with those observed in satellite data.

Sharma, O. P.↗
NASA NTRS1985

Upconversion of lower-hybrid waves by gyrating ion beams in a plasma

A gyrating ion beam, with ring shape distribution in velocity space, supports negative energy modes near the harmonics of beam gyrofrequency. An investigation of the nonlinear interaction of high-frequency lower-hybrid waves with the negative energy beam cyclotron mode is made. A nonlinear dispersion relation is derived for the coupled modes. It is shown that a gyrating ion beam frequency upconverts the lower-hybrid waves separated by harmonics of the beam gyrofrequency. This process of upconversion may thus provide a tool for the diagnostics of the gyrating ion beams. The process may also be important in the saturation of lower-hybrid waves observed in the auroral zone S3-3 satellite data.

Sharma, O. P.↗
NASA NTRS1984

Drift wave instabilities in a high beta multispecies plasma

The dispersion relation for drift-Alfven waves in two-component (cold and hot), high beta inhomogeneous multispecies plasma containing protons, oxygen, and sulfur ions is solved numerically. The magnetic field is assumed to have a gradient in simple slab geometry configuration. The plasma ion composition consists of H, O, S, SO2, and Na ions. The effect of heavy ions and multiple charge states on the growth rates has been numerically estimated for assumed ion compositions. The numerical calculations of growth rates also have been performed by using ion composition and plasma parameters based on Voyager spacecraft in the magnetospheres of Jupiter and Saturn.

Patel, V. L.↗
NASA NTRS1983

Stimulated Raman up-conversion of electromagnetic waves by a gyrating electron beam

A gyrating electron beam supports negative energy modes near the harmonics of electron-cyclotron frequency. An electromagnetic wave passing through such a beam parametrically up-converts into high-frequency electromagnetic modes separated from the pump frequency by the electron-cyclotron harmonics. The growth rate for this process varies directly as the oscillatory velocity of beam electrons caused by the pump and as square root of the beam density. It has a maximum at values of scattering angle close to 180 deg and is also implicitly dependent on the beam veocity and the cyclotron frequency of electrons. The effect of a cold electron component is to reduce the growth rate.

Sharma, O. P.↗
NASA NTRS1978

Interplanetary magnetic field variations and the electromagnetic state of the equatorial ionosphere

The Esq phenomena were selected in order to examine the effect of the interplanetary magnetic field (IMF) on the ionospheric plasma and to obtain insight into the interplanetary ionospheric coupling processes. January-March 1973 interplanetary magnetic field data of Explorer 43, Huancayo ionograms, and surface equatorial magnetograms were used. The IMF observations from Explorer 43 in the form of 15-sec averages were examined around the time of disappearance of the Esq. The IMF z-component was observed to change from a negative to a positive value before the disappearance of the Esq in four events where simultaneous data were available. The general explanation is that the induced electric field becomes westward from a previous eastward direction, coinciding with the IMF z-component reversal. Thus, just before the Esq disappears, the magnetosphere is subjected to the westward electric field. If this field is impressed to the low-latitude ionosphere, the resultant electric field in the equatorial ionosphere changes from eastward (westward) to westward (eastward) in the daytime (nighttime).

Patel, V. L.↗
NASA NTRS1975

Effect of interplanetary magnetic field on ionosphere over the magnetic equator

Large and quick changes of the latitude of the interplanetary magnetic field from its southward to northward direction are shown to be associated with the disappearance of the Es-q layer (Knecht, 1959) at the equatorial ionosphere during the daytime or with the reversal of E region horizontal and F region vertical electron drifts during both night and day. This phenomenon is suggested as the imposition of an electric field in the ionosphere in a direction opposite to that of the Sq electric field. The resultant electrostatic field on the equatorial ionosphere would be decreased or even reversed from its normal direction, resulting in the reduction of electron drift velocity. When the normal Sq field is over-compensated by the magnetospheric electric field, the electron drifts are reversed and the irregularities in the E region due to the cross-field instabilities are inhibited, resulting in the sudden disappearance of the Es-q layers.

Rastogi, R. G.↗
NASA NTRS1975

A study of geomagnetic storms

Twenty-one geomagnetic storm events during 1966 and 1970 were studied by using simultaneous interplanetary magnetic field and plasma parameters. Explorer 33 and 35 field and plasma data were analyzed on large-scale (hourly) and small-scale (3 min.) during the time interval coincident with initial phase of the geomagnetic storms. The solar-ecliptic Bz component turns southward at the end of the initial phase, thus triggering the main phase decrease in Dst geomagnetic field. When the Bz is already negative, its value becomes further negative. The By component also shows large fluctuations along with Bz. When there are no clear changes in the Bz component, the By shows abrupt changes at the main phase onet. On the small-scale behavior of the magnetic field and electric field (E=-VxB) studied in details for the three events, it is found that the field fluctuations in By, Bz and Ey and Ez are present in the initial phase. These fluctuations become larger just before the main phase of the storm begins. In the largescale behavior field remains quiet because the small scale variations are averaged out.

Patel, V. L.↗
NASA NTRS1975

Interplanetary field and plasma during initial phase of geomagnetic storms

Twenty-three geomagnetic storm events during 1966 to 1970 were studied by using simultaneous interplanetary magnetic field and plasma parameters. Explorer 33 and 35 field and plasma data were analyzed on large-scale (hourly) and small-scale (3 min.) during the time interval coincident with the initial phase of the geomagnetic storms. The solar-ecliptic Bz component turns southward at the end of the initial phase, thus triggering the main phase decrease in Dst geomagnetic field. The By component also shows large fluctuations along with Bz. When there are no clear changes in the Bz component, the By shows abrupt changes at the main phase onset. On the small-scale, behavior of the magnetic field and electric field were studied in detail for the three events; it is found that the field fluctuations in By, Bz and Ey and Ez are present in the initial phase. In the large-scale, the behavior field remains quiet because the small-scale variations are averaged out. It appears that large as well as small time scale fluctuations in the interplanetary field and plasma help to alter the internal electromagnetic state of the magnetosphere so that a ring current could causing a geomagnetic storm decrease.

Patel, V. L.↗
NASA NTRS1975

Interplanetary field and plasma during initial phase of geomagnetic storms

A study has been conducted of a large number of geomagnetic storms occurring during the period from 1966 to 1970. Questions of data selection are discussed and the large-scale interplanetary magnetic field during the initial phase is examined. Small-scale interplanetary fields during the initial phase are also considered, taking into account important features of small-scale variations in the interplanetary field and plasma for three storms. Details concerning 23 geomagnetic storms and the interplanetary magnetic field are presented in a table. A study of the initial phase of these storms indicates that in most of these events, the solar-ecliptic Z component of the interplanetary magnetic field turns southward when the main phase decrease begins.

Patel, V. L.↗
NASA NTRS1974

Micropulsation periods and the size of the magnetosphere

Using a set of 112 magnetopause locations selected from those obtained in recent years with the aid of Imp satellites on the dayside of the magnetosphere over 0600-1800 LT, extended results are presented on the problem of geomagnetic micropulsation periods in relation to magnetospheric parameters. These results include the finding that the two groups represented by micropulsation period ranges from 20 to 40 sec and from 40 to 80 sec, respectively, have a completely different dependence on the radial distance of the magnetopause from satellite experiments.

Patel, V. L.↗
NASA NTRS1973

Interplanetary magnetic field and geomagnetic Dst variations.

The interplanetary magnetic field has been shown to influence the ring current field represented by Dst. Explorer 28 hourly magnetic field observations have been used with the hourly Dst values. The moderate geomagnetic storms of 60 gammas and quiet-time fluctuations of 10 to 30 gammas are correlated with the north to south change of the interplanetary field component perpendicular to the ecliptic. This change in the interplanetary field occurs one to three hours earlier than the corresponding change in the Dst field.

Patel, V. L.↗
NASA NTRS1972

Sudden impulses in the geomagnetotail and the vicinity.

Explorer 33 magnetic field observations have been analyzed to study the propagation of sudden impulses (si) in the geomagnetic tail and the magnetosheath. Average speed of propagation in these regions is 760 km per sec with a range of 550-1230 km per sec. Most of the si's have a propagation speed which is close to that of the shock wave observed in this region. Positive si is shown to have a positive change in the tail or magnetosheath and a negative si has a similar negative change. Nine out of thirteen correlated si's have their origin in the solar wind continuities, while four si's have possible origin in the perturbations of tail or magnetosheath field.

Patel, V. L.↗
NASA NTRS1971

Sudden impulses in the geomagnetotail and the vicinity

Explorer 33 magnetic field observations were analyzed to study the propagation of sudden impulses (SI) in the geomagnetic tail and the magnetosheath. Average speed of propagation in these regions is 760 km per sec with a range of 550 to 1230 km per sec. Most of the SI's have propagation speed close to that of the shock wave observed in this region. Positive SI is shown to have a positive change in the tail or magnetosheath and a negative SI has a similar negative change. Nine out of thirteen correlated SI's have their origin in the solar wind continuities, while four SI's possibly originate in the perturbations of the tail or magnetosheath field. It was found that the SI propagation is not confined to the tail region but has correlated effects in the magnetosheath. The propagation region studied was in 2000 to 0200 local time sector.

Patel, V. L.↗