Anita Kullen

Solar wind dependence of the occurrence and motion of polar auroral arcs : A statistical study

[1] Polar UV images from a 3-month period in winter 1998-1999 are used for a statistical study of polar arcs. The study covers all auroral arcs that are located poleward of the northern auroral ova ...

On the relation between solar wind, pseudobreakups, and substorms

A statistical study of pseudobreakups and substorms is performed using Polar UV images from a 3-month period in winter 1998-1999. Data from the ACE solar wind monitor are examined in order to deter ...

The connection between transpolar arcs and magnetotail rotation

Recent observations show that the evolution of transpolar arcs (TPAs) are often associated with a sign change of the dawn-dusk component of the interplanetary magnetic field (IMF). It is known tha ...

Statistical investigation of Kelvin-Helmholtz waves at the magnetopause of Mercury

A large study of Kelvin-Helmholtz (KH) waves at the magnetopause of Mercury covering 907 days of data from the MErcury Surface Space ENvironment GEochemistry Ranging spacecraft have resulted in 146 ...

Occurrence and properties of substorms associated with pseudobreakups

We investigate how substorms with and without growth-phase pseudobreakups are affected by solar wind and ionospheric conditions. The study is based on 874 events identified with Polar UVI. An AE in ...

Magnetic forces associated with bursty bulk flows in Earth's magnetotail

We present the first direct measurements of magnetic forces acting on bursty bulk flow plasma in the magnetotail. The magnetic forces are determined using Cluster multispacecraft measurements. We a ...

Dayside reconnection under interplanetary magnetic field By‐dominated conditions: The formation and movement of bending arcs

Based upon a survey of global auroral images collected by the Polar Ultraviolet Imager, Kullen et?al. (2002) subdivided polar cap auroral arcs into a number of categories, including that of “bending” arcs. We are concerned with those bending arcs that appear as a bifurcation of the dayside auroral oval and which subsequently form a spur intruding into the polar cap. Once formed, the spur moves poleward and antisunward over the lifetime of the arc. We propose that dayside bending arcs are ionospheric signatures of pulses of dayside reconnection and are therefore part of a group of transient phenomena associated with flux transfer events. We observe the formation and subsequent motion of a bending arc across the polar cap during a 30 min interval on 8 January 1999, and we show that this example is consistent with the proposed model. We quantify the motion of the arc and find it to be commensurate with the convection flows observed by both ground-based radar observations and space-based particle flow measurements. In addition, precipitating particles coincident with the arc appear to occur along open field lines, lending further support to the model.

Seasonal dependence and solar wind control of transpolar arc luminosity

[1] The influence of the solar wind and the interplanetary magnetic field (IMF) on the luminosity of transpolar arcs (TPAs) is examined by taking into account seasonal effects. The study focuses on those transpolar arcs that appear after an IMF By sign change during steady northward IMF. It includes 21 northern hemisphere events identified in a previous study from global UV images taken by the Polar spacecraft between 1996 and 2000. Sorting the TPA events by sign of the Earth dipole tilt we find that the TPAs which appear in the dark hemisphere are on average much weaker than TPAs in the sunlit hemisphere. For the dark hemisphere events, no clear correlation between solar wind parameters and TPA luminosity is found. However, in the sunlit hemisphere, a clear dependence on solar wind and IMF conditions is seen. The TPA brightness is strongly influenced by IMF magnitude, northward IMF Bz and solar wind speed. A weak, negative correlation with the ion density is found. The TPA luminosity in the sunlit hemisphere is much more strongly controlled by the magnetic energy flux than by the kinetic energy flux of the solar wind. This explains the absence of transpolar arcs for the two By sign change cases for positive dipole tilts with lowest magnetic energy flux values. The strong influence of the Earth dipole tilt on the transpolar arc luminosity appears due to the dependence of the ionospheric conductivity on solar EUV emissions.

The influence of IMF By on the mapping between the Earth's magnetotail and its ionosphere

It is well known that the dawn-dusk component of the interplanetary magnetic field (IMF) creates asymmetries in the magnetotail. The main effects are a weak nonuniform penetration of IMF B y into the tail and a rotation of the whole tail current sheet. In this study we modify the Tsyganenko (1989) model to simulate the IMF B y induced tail effects. By field line mapping we examine how different regions of the magnetosphere and ionosphere are influenced by the IMF B y tail effects. The plasma sheet is shown to rotate around the central axis of the magnetotail. The mapping from the plasma sheet to the ionosphere for nonzero IMF B y shows a change in the geometry of the auroral oval. The whole oval rotates, a local bulge occurs near midnight and the thickness of the oval is different in the morning and the evening sector. Simple geometrical considerations show that the tilting of the field lines caused by IMF B y leads to a rotation of the oval, whereas the rotation of the tail current sheet is reponsible for the thickness variations along the oval. These results fit very well with the observed auroral distribution for an IMF with a dawnward or duskward direction.

The statistical difference between bending arcs and regular polar arcs

In this work, the Polar UVI data set by Kullen et al. (2002) of 74 polar arcs is reinvestigated, focusing on bending arcs. Bending arcs are typically faint and form (depending on interplanetary magnetic field (IMF) By direction) on the dawnside or duskside oval with the tip of the arc splitting off the dayside oval. The tip subsequently moves into the polar cap in the antisunward direction, while the arcs nightside end remains attached to the oval, eventually becoming hook-shaped. Our investigation shows that bending arcs appear on the opposite oval side from and farther sunward than most regular polar arcs. They form during By-dominated IMF conditions: typically, the IMF clock angle increases from 60 to 90° about 20?min before the arc forms. Antisunward plasma flows from the oval into the polar cap just poleward of bending arcs are seen in Super Dual Auroral Radar Network data, indicating dayside reconnection. For regular polar arcs, recently reported characteristics are confirmed in contrast to bending arcs. This includes plasma flows along the nightside oval that originate close to the initial arc location and a significant delay in the correlation between IMF By and initial arc location. In our data set, the highest correlations are found with IMF By appearing at least 1–2?h before arc formation. In summary, bending arcs are distinctly different from regular arcs and cannot be explained by existing polar arc models. Instead, these results are consistent with the formation mechanism described in Carter et al. (2015), suggesting that bending arcs are caused by dayside reconnection.