摘要:

Thermospheric concentrations of neutral atomic hydrogen near and below theFpeak are directly related to H+, O+, and atomic oxygen concentrations through the charge exchange equilibrium that is established between hydrogen and oxygen at these altitudes. This chemical relationship, together with in situ measurements of ionospheric and neutral atmospheric concentrations by instrumentation on board the Atmosphere Explorer C satellite, is utilized to investigate properties of neutral hydrogen at altitudes below 200 km where vertical diffusion strongly affects the hydrogen distribution. Data are discussed for a set of satellite orbits during quiet geomagnetic and solar conditions in February 1974; the resultant altitude variation of the derived hydrogen concentrations applies specifically to early afternoon at low latitudes between the winter solstice and the spring equinox. A hydrogen concentration of (2.55 ± 0.5) × 105atoms/cm³ is observed for these conditions at 300 km. At lower altitudes the concentration profiles are interpreted in terms of vertical hydrogen flow. The resultant daytime flux in the thermosphere is estimated to be (3.2 ± 1.0) × 108atoms/cm² s. The present observations thus support theoretical estimates and model calculations of large hydrogen flows upward from the region below 100 km. They also support the concept of daytime thermospheric loss processes of greater magnitude than the traditional evaporative escape mech

ISSN:0148-0227    

DOI:10.1029/JA081i016p02677

年代:1976

数据来源: WILEY

摘要:

The response of the plasma mantle to the orientation of the interplanetary magnetic field (IMF) has been studied by correlating Heos 2 plasma and Imp 6 magnetic field data. The mantle is nearly always present when the IMF has a southward component and often also when the field has a weak northward component. In addition, the mantle appears increasingly thicker with greater southward components. On the other hand, the mantle is thin or missing (from the region where it is normally found) when the average IMF has a strong northward component. This result supports the idea that polar cap convection plays a dominant role in the formation of the plasma mantle: Mantle plasma originates in the magnetosheath, enters the magnetosphere through the day side polar cusps, and is transported across the cusp to the night side by means of a convection electric field whose magnitude is controlled by the orientation of the IMF.

ISSN:0148-0227    

DOI:10.1029/JA081i016p02687

年代:1976

数据来源: WILEY

摘要:

A study has been made of the conditions under which ≃2‐ to 6‐kHz VLF signals transmitted from Siple, Antarctica (L≃ 4), are observed at the conjugate ground station Roberval, Canada, following ducted propagation through the magnetosphere. In 1973 and 1974, signals detected from visual inspection of spectrographic records were present on roughly 20% of the days, typically for ≃4 of 8 observing hours. As had been expected, the observations appeared sensitive to conditions of whistler propagation and of wave growth in the magnetosphere. Signal observations were most frequent during quieting following magnetic disturbance. A diurnal activity peak was found at dawn, and day side activity levels were generally higher than those at night. Signal travel times were compared to the dispersion properties of whistlers recorded at Siple and Roberval during July–August 1973 and September–November 1974. The signal path equatorial radii were found to be concentrated in the range 3.5–4.5RE, which is centered roughly on the Siple/Roberval field lines. At ionospheric heights this range corresponds to a latitude interval of ≃3°. Multipath propagation was frequently observed, but a single path was usually prominent in terms of received amplitude. During relatively quiet conditions the paths were located within the outer plasmasphere; during moderate disturbance they frequently appeared in the region of steep plasmapause

ISSN:0148-0227    

DOI:10.1029/JA081i016p02692

年代:1976

数据来源: WILEY

摘要:

Explorer 45 (S³‐A) measurements during the recovery phase of a moderate magnetic storm have confirmed that the charge exchange decay mechanism can account for the decay of the storm time proton ring current. The moderate magnetic storm of February 24, 1972, was selected for study, since a symmetric ring current had developed and effects due to asymmetric ring current losses could be eliminated. In this study it was found that after the initial rapid decay of the proton flux, which is a consequence of the dissipation of the asymmetric ring current, the equatorially mirroring protons in the energy range 5–30 keV decayed throughout theLvalue range of 3.5–5.0 at the charge exchange decay rate calculated by Liemohn (1961). After several days of decay, the proton fluxes reached a lower limit where an apparent equilibrium was maintained, between weak particle source mechanisms and the loss mechanisms, until fresh protons were injected into the ring current region during substorms. While other proton loss mechanisms may also be operating, the results indicate that charge exchange is more than sufficient as a particle loss mechanism for the storm time proton ring current decay and that this mechanism must be considered in all studies involving the loss of proton ring current par

ISSN:0148-0227    

DOI:10.1029/JA081i016p02701

年代:1976

数据来源: WILEY

摘要:

A jet aircraft, flying from Goose Bay, Labrador, to Fairbanks, Alaska, made auroral observations at nearly constant magnetic local time (∼2100 MLT) in the auroral zone while a Vela satellite passed through the plasma sheet atr≃ 18REat nearly the same magnetic local time. Comparisons of data from the two locations provide further confirmation of the ‘poleward leap’ of the auroral electrojet which occurs in a late phase of an auroral substorm and is associated with a rapid tailward motion of an X‐type neutral line in the magnetotail. The poleward leap is a distinctive feature of substorm evolution and is not simply the superposition of a new substorm on the recovery phase of a preceding substorm. It probably marks the sudden transition of the magnetotail from one quasi‐stable configuration to another more stable one. Onset of a substorm expansive phase brings about a change of tail magnetic field from a configuration that is extremely taillike, with field lines from λm≲ 66° stretching to the Vela orbit, to one that is much less taillike, with field lines from λm≳ 70° not stretching as fa

ISSN:0148-0227    

DOI:10.1029/JA081i016p02709

年代:1976

数据来源: WILEY

摘要:

Scalar and vector velocity differences between helium and hydrogen ions in the solar wind measured with the Los Alamos plasma analyzers on Imp 6 and Imp 7 are presented and interpreted. From the bulk speeds and azimuthal components of flow direction for both types of ions the ecliptic projection of the helium to hydrogen velocity difference vectorvαpis determined. Short‐term and long‐term average properties of this vector, at times contrasted with the more usually reported bulk speed difference υα− υp, are principal subjects of this paper. The vector is usually found aligned with the proton heat flux vectorQpand therefore is directed predominantly outward from the sun. Sometimes preceding high‐speed streams and during fluctuations the vector is directed back toward the sun; i.e., υp>υα. There is a statistical correlation between the magnitude ofvαpand the wind speed; empirically, |vαp| ≃ 0.08υp− 23 for υp≥ 420 km s−1. During high‐speed streams, |vαp| is often 25 or 30 km s−1and on rare occasions as large as 80 km s−1, while at lower wind speeds it is<10 km s−1. In high‐speed streams or in other magnetically disturbed regions the vector difference magnitude often remains nearly constant, while the bulk speed difference undergoes large changes as the difference vector rotates in alignment withB. The underlying physical causes of the He and H

ISSN:0148-0227    

DOI:10.1029/JA081i016p02719

年代:1976

数据来源: WILEY

摘要:

The cross‐Lmotions of the magnetospheric plasma are inferred from measured time variations of the minimum value ofBalong whistler paths. Most of the observed paths were in the range 3.5

ISSN:0148-0227    

DOI:10.1029/JA081i016p02728

年代:1976

数据来源: WILEY

摘要:

The daily variations in ionospheric drift velocities are examined from incoherent scatter measurements at Millstone Hill. The data with ∑Kp>24 behave differently from those with low magnetic activity and basically follow the convection pattern but have large day‐to‐day variations. The influence of the magnetic conjugate point is discussed, and solar cycle variations are examined in conjunction with geomagnetic variations. Ionospheric currents calculated by using a semidiurnal neutral wind model are in good agreement with ground‐based magnetograms for low magnetic activity, but theEregion neutral wind model appears to be applicable only to th

ISSN:0148-0227    

DOI:10.1029/JA081i016p02737

年代:1976

数据来源: WILEY

摘要:

The linear dispersion relation for electromagnetic waves is investigated for homogeneous plasmas normally found in the solar wind near 1 AU, including a secondary proton beam of varying strength and drift speed parallel to the ambient magnetic field. Three instabilities are found when the drift speed of the beam approaches the Alfvén speedCA: (1) field‐aligned magnetosonic, (2) oblique magnetosonic, and (3) oblique Alfvén modes. Linear growth rates as a function of β = 8πnTp/B0², drift speed,Te/Tp, proton and electron temperature anisotropies, and electron heat flux have been studied. Results show that beam‐driven magnetosonic and Alfvén waves are likely to be important in the solar wind if the fraction of protons in the beam exceeds 5% and the drift speed exceeds ∼0.8CAwhen β ≲ 0.1 (or ∼1.

ISSN:0148-0227    

DOI:10.1029/JA081i016p02743

年代:1976

数据来源: WILEY

摘要:

Using the Mariner 5 solar wind plasma and magnetic field data, we present observations of field‐aligned Suprathermal proton velocity distributions having pronounced high‐energy shoulders. These observations, similar to the interpenetrating stream observations of Feldman et al. (1974), are clear evidence that such proton distributions are interplanetary rather than bow shock associated phenomena. Large Alfvén speed is found to be a requirement for the occurrence of suprathermal proton distributions; further, we find the proportion of particles in the shoulder to be limited by the magnitude of the Alfvén speed. It is suggested that this last result could indicate that the proton thermal anisotropy is limited at times by wave‐particle inter

ISSN:0148-0227    

DOI:10.1029/JA081i016p02750

年代:1976

数据来源: WILEY