Empirical Magnetic Field Research Articles

Magnetic energy storage and the nightside magnetosphere‐ionosphere coupling

The change in the magnetic energy stored in the Earth's magnetotail for the January 14–15 1988 magnetic cloud event is investigated using an empirical magnetic field model. The results are used to calculate the two normal modes contained in the low‐dimensional global model called WINDMI for the solar wind driven magnetosphere‐ionosphere system. For that event, we find that the storage of magnetic energy in the geotail increases most rapidly (by 17%) in the phase where the IMF turns from northward to the dawn‐dusk direction and then by an additional 5% in the phase where the IMF rotates from dawn‐dusk to southward. The coupling of the magnetosphere‐ionosphere (MI) through the night side region 1 current loop transfers power to the ionosphere through two modes: a fast (period of minutes) oscillation and a slow (period of one hour) geotail cavity mode. The solar wind drives both modes in the substorm dynamics.

Erosion of the inner magnetosphere during geomagnetic storms

Abstract. Using the empirical magnetic field model dependent on the Dst index and solar wind dynamic pressure, we calculated the behaviour of the contour B = Bs in the equatorial plane of the magnetosphere where Bs is the magnetic field in the subsolar point at the magnetopause. The inner domain of the magnetosphere outlined by this contour contains the bulk of geomagnetically trapped particles. During quiet time the boundary of the inner magnetosphere passes at the distance ~10 RE at noon and at ~7 RE at midnight. During very intense storms this distance can be reduced to 4–5 RE for all MLT. The calculation results agree well with the satellite measurements of the magnetopause location during storms. The ionospheric projection of the B = Bs contour calculated with the Euler potential technique is close to the equatorward edge of the auroral oval.

Erosion of the inner magnetosphere during geomagnetic storms

Using the empirical magnetic field model dependent on the Dst index and solar wind dynamic pressure, we calculated the behaviour of the contour B = Bs in the equatorial plane of the magnetosphere where Bs is the magnetic field in the subsolar point at the magnetopause. The inner domain of the magnetosphere outlined by this contour contains the bulk of geomagnetically trapped particles. During quiet time the boundary of the inner magnetosphere passes at the distance ~10 RE at noon and at ~7 RE at midnight. During very intense storms this distance can be reduced to 4–5 RE for all MLT. The calculation results agree well with the satellite measurements of the magnetopause location during storms. The ionospheric projection of the B = Bs contour calculated with the Euler potential technique is close to the equatorward edge of the auroral oval.

Comparison of empirical magnetic field models and global MHD simulations: The near‐tail currents

The tail currents predicted by empirical magnetic field models and global MHD simulations are compared. It is shown that the near‐Earth currents obtained from the MHD simulations are much weaker than the currents predicted by the Tsyganenko models, primarily because the ring current is not properly represented in the simulations. On the other hand, in the mid‐tail and distant tail the lobe field strength predicted by the simulations is comparable to what is observed at about 50RE distance, significantly larger than the very low lobe field values predicted by the Tsyganenko models at that distance. Ways to improve these complementary approaches to model the actual magnetospheric configuration are discussed.

Modeling the magnetic effects of field‐aligned currents

In this paper, I present a newly developed model of the magnetic effects of field‐aligned currents. The magnetic field of a large‐scale field‐aligned current system is obtained by summing the magnetic fields of a large number of finite length cylindrical current elements. The resulting magnetic field is valid inside of, near and far away from the model field‐aligned currents, both near the ionosphere and in the magnetosphere. The modeling technique allows for the closure of the field‐aligned currents along any specified path. In order to investigate the effects of field‐aligned currents on the nightside magnetospheric magnetic field and on mapping between the night‐side magnetosphere and ionosphere, I add the effects of region I and II field‐aligned currents to an empirical magnetic field model. Using this model, I demonstrate the following three things: (1) reasonable amounts of field‐aligned current can cause the ionospheric footprint of a point at geosynchronous orbit to shift in local time by one hour or more; (2) depending on how they are closed, field‐aligned currents can cause Bz in the equatorial plane, near midnight, either to increase or to decrease; (3) CPS field lines can cross the equatorial plane 10 RE or more further from the noon‐midnight meridian than they would if field‐aligned current effects were not included, a result that also depends on how the currents are closed.

Uses and limitations of the Tsyganenko magnetic field models

When exploring a new country, one needs a good map. The Earth's magnetosphere is such a country, and its “maps” are models of the geomagnetic field. In essence they are mathematical prescriptions implemented by computer which, given (x,y,z) of a point and other supporting information, return the point's expected magnetic components (Bx, By, Bz) [Tsyganenko, 1990].At present, the empirical magnetic field models by Tsyganenko [1987, 1989a, c; denoted henceforth as T87 and T89] are often used in various studies of the Earth's magnetosphere. Stern [1990] has devised modifications of T87 with current‐free tail lobes and with a plasma sheet that warps in response to dipole tilt [Peredo and Stern, 1991].

Eddy current compensation by direct field detection and digital gradient modification

Abstract We demonstrate an empirical magnetic field gradient waveform optimization technique which compensates for eddy current distortions and nonideal gradient amplifier performance. The desired gradient response is used as a trial function, and a pickup coil in the magnet bore detects the actual pulsed gradient. The temporal behavior of the gradient is digitized and recorded with the existing NMR spectrometer hardware and is used for the calculation of a new trial function table. The method and some typical results as well as some practical considerations are described.

Magnetospheric plasma pressures in the midnight meridian: Observations from 2.5 to 35 RE

Plasma pressure data from the ISEE 2 fast plasma experiment (FPE) were statistically analyzed to determine the plasma sheet pressure versus distance in the midnight local time sector of the near‐earth (12–35 RE) magnetotail plasma sheet. The observed plasma pressure, assumed isotropic, was mapped along model magnetic field flux tubes (obtained from the Tsyganenko and Usmanov [1982] model) to the magnetic equator, sorted according to magnetic activity, and binned according to the mapped equatorial location. In regions (L ≳ 12 RE) where the bulk of the plasma pressure was contributed by particles in the energy range of the FPE (70 eV to 40 keV for ions), the statistically determined peak plasma pressures vary with distance similarly to previously determined lobe magnetic pressures (i.e., in a time‐averaged sense, pressure balance normal to the magnetotail magnetic equator in the midnight meridian is maintained between lobe magnetic and plasma sheet plasma pressures). Additional plasma pressure data obtained in the inner magnetosphere (2.5 < L < 7) by the Explorer 45, ATS 5, and AMPTE CCE spacecraft supplement the ISEE 2 data. Estimates of plasma pressures in the “transition” region (7–12 RE), where the magnetic field topology changes rapidly from a dipolar to a tail‐like configuration, are compared with the observed pressure profiles. The quiet time “transition” region pressure estimates, obtained previously from inversions of empirical magnetic field models, bridge observations both interior to and exterior to the “transition” region in a reasonable manner. Quiet time observations and estimates are combined to provide profiles of the equatorial plasma pressure along the midnight meridian between 2.5 and 35 RE.

Projection of auroral intensity contours into the magnetosphere

Abstract Isointensity contours of 630 nm auroral emission are traced into the magnetosphere, using two different empirical magnetic field models, the Mead-Fairfield model, and the Hedgecock-Thomas model. The auroral data are for a specific ISIS-II satellite pass, and so the starting points are expressed in geographic latitude and longitude coordinates, at a specific universal time. The magnetic field models are constructed from satellite magnetometer measurements, and those used correspond to magnetically quiet times. The projections are found to agree reasonably well with direct plasma measurements of the plasma sheet. The projections of the dayside contour connect to widely different regions of the magnetosphere, providing an interpretation that is consistent with observations of the dayside aurora. It is concluded that field line projections of the aurora into the magnetosphere using these models is a valid procedure, but only under quiet-time conditions.

Facular line profiles and facular models

view Abstract Citations (56) References (43) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Facular line profiles and facular models Chapman, G. A. Abstract Profiles for lines near 5250 A have been calculated from facular models and compared with observed profiles. The observations are from photographic and photoelectric spectra. The atomic parameters are determined by comparison of calculated profiles with those observed for the quiet sun. The effects of finite spatial resolution and a magnetic field are required to obtain agreement with observed facular line profiles. The empirical magnetic field deduced by matching observed and calculated profiles is reasonably consistent with that in the facular models. The main purpose of this paper is to show that by including the effects of scattered photospheric light and a strong magnetic field, reasonable agreement can be achieved between observed and calculated facular line profiles. Results are also presented for the wing of the Ca II K line, showing line profiles and effective filtergram contrasts for several facular models. It is concluded that observations made with weak Fraunhofer lines are not very useful for discrimination among facular models, particularly without simultaneous magnetic-field measurements. The K line may offer a better discriminator. The facular model presently favored has a magnetic field strength of about 1500 gauss at the surface of the photosphere. Publication: The Astrophysical Journal Supplement Series Pub Date: January 1977 DOI: 10.1086/190417 Bibcode: 1977ApJS...33...35C Keywords: Faculae; Light Scattering; Line Spectra; Solar Magnetic Field; Solar Spectra; Astronomical Models; Astronomical Photography; Curve Fitting; Filtergrams; Fine Structure; Fraunhofer Lines; Graphs (Charts); Magnetic Flux; Astrophysics full text sources ADS |