The aurora at quiet magnetospheric conditions: Repeatability and dipole tilt angle dependence

Abstract

Is there a magnetospheric ground state? Do the position and size of the auroral oval depend on the magnetic dipole tilt angle at quiet magnetospheric conditions? In order to address these questions, northern hemisphere images of the aurora at 1356 Å, obtained by Polar BEAR at solar minimum (beginning of 1987), were related to high temporal resolution IMP 8 measurements of the interplanetary magnetic field, to solar wind velocity, and to the ground‐based activity index Kp. The first problem was addressed by a two‐dimensional correlation study of the repeatability of auroral emissions in corrected geomagnetic space at conditions of minimum energy transfer from the magnetosphere. The correlation measure of auroral images was 0.6‐0.85. Error simulations indicate that given the uncertainties in pixel position and intensity, the maximum expected value of the correlation measure is 0.65‐0.9. The notion of a ground state magnetosphere is therefore supported by our data. Repeatability was found at the same level regardless of time or reconfigurations of the magnetosphere between images and independent of magnetic time sector. The second problem was addressed by relating latitudinal shifts of the aurora with dipole tilt angle without resorting to auroral boundary specification. Our data indicate that the latitude of the continuous aurora is related to the dipole tilt angle at quiet magnetospheric conditions. In the winter hemisphere a 10° increase in the dipole tilt angle causes a 1° decrease (increase) in the latitude of auroral emissions at noon (midnight). The magnetic local time distribution of the latitudinal shifts with dipole tilt angle support a simple model in which the dipole tilt angle determines the position of the center of the auroral circle along the magnetic meridian 1320‐0120 MLT (for IMF By positive) and does not affect its radius.

Open‐closed field line boundary position: A parametric study using an MHD model

“…The physical conditions leading to steady magnetospheric convection events are not yet entirely understood; however, in a recent paper O'Brien et al [2002] describe some of the statistics of their occurrence. The repeatability of auroras under similar quiet magnetospheric conditions was demonstrated by Oznovich et al [1993], which lends further credibility to the concept of steady state in the context of our study. It is such cases of steady magnetospheric convection that are primary applications of this study.…”

“…However, even for the extreme values of the dipole tilt, the differences between the open‐closed field line boundaries are quite small, of the order of 1°–2° of magnetic latitude. This change in the location of the OCB is of the same order of magnitude as that obtained by Oznovich et al [1993] in their study of seasonal effects on auroral emissions.…”

“…This variation adds an additional dimension to the parameter space of the problem studied in this paper. It is known, however, that the OCB is centered, to a first order, relative to the geomagnetic pole [ Feldstein and Starkov , 1970; Oznovich et al , 1993]. In particular, Oznovich et al [1993] experimentally found that if the dipole tilt angle changes by 10°, then the latitude of the auroral emissions changes only by about 1°.…”

Interhemispheric observations of emerging polar cap asymmetries

“…The only earlier study that has utilized conjugate images from IMAGE and Polar to study the entire auroral ovals, Stubbs et al [2005], showed that the ovals (as represented by a best fit circle) were displaced in accordance with the expected B y and B x effects [e.g., Cowley et al , 1991] and dipole tilt effect [e.g., Oznovich et al , 1993]. Changes in asymmetry were seen to be directly related to changes in IMF orientation, consistent with newly opened field lines being subject to magnetic stresses [ Jørgensen et al , 1972].…”

<title>Recent advances in atmospheric ultraviolet remote sensing</title>

“…This work is described by Oznovich, et a!., 1993aOznovich, et a!., , 1993b. The Polar BEAR images were also used in conjuction with ground cameras, satellite beacons, and the Sondrestrom incoherent scatter radar to characterize irregularity regions leading to scintilations in propagated radio waves, Robinson, R., et a!., 1992; Doe, et a!., 1993; Basu, S., et a!., 1994.…”