Effects of the interplanetary magnetic field y component on the dayside aurora
A dawn–dusk asymmetry in many high-latitude ionospheric and magnetospheric phenomena, including the aurora, can be linked to the east–west (y) component of the interplanetary magnetic field (IMF). Owing to the scarcity of observations in the Southern Hemisphere, most of the previous findings are associated with the Northern Hemisphere. It has long been suspected that if the IMF By component also produces a dawn–dusk asymmetry and/or a mirror image in the Southern Hemisphere as predicted by some theories. The present study explores the effect of the IMF By component on the dayside aurora from both hemispheres by analyzing the auroral emission data from the Global UltraViolet scanning spectrograph Imager on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics mission spacecraft from 2002 to 2007. The data set comprises 28,774 partial images of the northern hemispheric oval and 29,742 partial images of the southern hemispheric oval, allowing for a statistical analysis. It is found that even though auroras in different regions of the dayside oval respond differently to the orientation of the IMF By component, their responses are opposite between the two hemispheres. For example, at ~ 1400–1600 MLT in the Northern Hemisphere, where the so-called 1500 MLT auroral hot spots occur, peak auroral energy flux is larger for negative IMF By comparing to positive IMF By. The response is reversed in the Southern Hemisphere. The present study also suggests that the total energy flux does not change with the IMF By orientation change. This result is consistent with a larger (smaller) convection vortex in the postnoon sector for IMF By < 0 (By > 0) resulting from anti-parallel merging.
KeywordsDayside aurora Hemispheric asymmetry TIMED GUVI FUV
altitude adjusted corrected geomagnetic
interplanetary magnetic field
International Satellites for Ionospheric Studies
Global Ultraviolet Imager
magnetic local time
Space Physics Data Facility
solar zenith angle
Thermosphere Ionosphere Mesosphere Energetics and Dynamics
The auroral oval, when viewed from space, is a continuous ring of luminosity circling the magnetic pole and covering both nightside and dayside. Auroras that occur on dayside are considered distinct from their nightside counterpart (e.g., Akasofu and Kan 1980; Meng and Lundin 1986). The statistical precipitation energy flux into the auroral oval was first derived by Newell et al. (1996) using particle precipitation data from the Defense Meteorological Satellite Program (DMSP) satellites and later by Liou et al. (1997) using satellite imaging data in far ultraviolet (FUV) from the polar spacecraft. These studies demonstrated two local intensity maxima and one intensity minimum in the dayside auroral oval. The region near the midday is typically the weakest in auroral intensity and was named “the midday gap” (Dandekar and Pike 1978). Observations with multispectral imaging from the International Satellites for Ionospheric Studies 2 (ISIS-2) satellite suggest that the midday gap is a region of intensity minimum (Murphree et al. 1980). It features magnetosheath-like soft electron precipitation and thus is associated with the cusp (Meng 1981). A few hours east of the midday between ~ 1400 and 1600 MLT (magnetic local time) is a region where most intense auroral activity occurs on dayside (e.g., Cogger et al. 1977) and is often called the 1500 MLT auroral hot spot. Sometimes periodic forms of auroras appear in the afternoon sector (e.g., Lui et al. 1989). The boundary plasma sheet is found to be the major source of the brightest auroral hot spots (Liou et al. 1999). The morning warm spot, which is less intense than the 1500 MLT hot spot, is located between 0600 and 1000 MLT and is associated with sub-keV electron precipitation (e.g., Newell et al. 1996).
The morphology of dayside auroras is believed to be controlled by the IMF orientation because solar wind energy enters the magnetosphere mainly through magnetic merging between the interplanetary magnetic field (IMF) and magnetospheric field on the dayside magnetopause. For example, the expansion and contraction of the auroral oval/polar cap are associated with the north–south (Bz in Geocentric Solar Magnetospheric (GSM) coordinate) component of the IMF (e.g., Holzworth and Meng 1975) and the dawn–dusk shift of the polar cap center for different signs of IMF (Holzworth and Meng 1984). This has been interpreted by the open magnetosphere model (e.g., Cowley 1981).
In addition, the y-component of IMF may result in a hemispheric dawn-dusk asymmetry in the auroral particle precipitation. The anti-parallel merging theory (Reiff and Burch 1985) predicts the merging site in regions of highest magnetic shear at the magnetopause. As shown in Figure 1, for a southward and large positive (negative) y-component of IMF, the merging site moves to dawn (dusk) in the Northern Hemisphere and dusk (dawn) in the Southern Hemisphere. This hemispheric dawn-dusk asymmetry in the merging site can lead to a hemispheric dawn-dusk asymmetry of the magnetospheric convection and field-aligned currents that produce a hemispheric dawn-dusk asymmetry in the dayside aurora."
The Viking FUV imaging system provided the first snapshot auroral images on global scales and led to a number of studies of dayside auroral activity in association with IMF By. For example, Murphree et al. (1981) reported that during non-substorm periods, discrete auroras near 1400 MLT are observed when the IMF By component is often negative. Vo and Murphree (1995) found that dayside auroral hot spots observed in the Viking UV images usually in the afternoon sector occur mostly during negative IMF By and have a less pronounced dependence on IMF Bz. Trondsen et al. (1999) examined auroral images from Viking UV imager observations acquired between March 1986 and November 1986 and concluded that increased dayside auroral intensity and the generation of narrow continuous auroral oval around noon are often associated with negative IMF By rather than positive IMF By. Based on ground-based meridian scanning photometer data, Karlson et al. (1996) reported an asymmetry in the prenoon–postnoon auroral occurrence distribution caused by the IMF By polarity under southward IMF conditions—auroral events predominantly occurring in the postnoon (prenoon) sector for IMF By < 0 (By > 0).
In summary, these previous studies have commonly suggested that a negative IMF By component favors the occurrence of the postnoon dayside auroras. This is supported by multi-spectral observations of the dayside aurora from the ground (Hu et al. 2012). They found that statistically auroral intensity at 630.0 nm increases monotonically with the north–south component of the solar wind electric field, with a larger (smaller) increasing rate in the postnoon than in the prenoon oval for IMF By < 0 (By > 0). However, contradictive reports exist. For example, Liou et al. (1998) analyzed the 4-month worth of global auroral images from polar and concluded that the auroral power deposited into the afternoon (1300–1800 MLT) sector of the northern oval is linearly proportional to the magnitude of IMF By component, meaning that the magnitude of IMF By is the controlling parameter. More recently, Yang et al. (2013) analyzed the same data from polar for one full year but found no significant IMF By effect on the dayside auroral intensity.
In this study, observations of the Northern and Southern auroras from the Global Ultraviolet Imager (GUVI) (Paxton et al. 1999) on board NASA’s Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics (TIMED) satellite during the period of 2002–2007 will be analyzed. The TIMED spacecraft was launched in the December of 2001 into in a near circular, Sun-synchronous orbit with an averaged altitude of ~ 612 km and a 74.07° inclination angle. A small (~ 3°/day) precession of the TIMED spacecraft enables a full local-time coverage every 4 months. GUVI is a spectrometer in far ultraviolet (FUV) wavelengths performing limb-to-limb (cross-track) scans every 15 s along the satellite orbit. The field-of-view (FOV) of the GUVI is capable of producing a swath of image of ~ 108 km along-track and ~ 2500 km cross-track. With the nominal orbital period of ~ 97 min, TIMED circles the Earth ~ 15 times a day, which allows GUVI to cover the entire Earth, at least in the polar region, daily.
The GUVI system was operated in spectral and image modes. We will use the image mode data for the present study because of its wider spatial coverage and more abundant. In this mode, auroras were imaged in five wavelengths in the FUV spectrum, among which the Lyman–Birge–Hopfield (LBH) long band will be analyzed here. The image mode data are available from February 2002 to November 2007. A total of 27,861 and 27,633 images of Northern and Southern Hemisphere, respectively, are available for the present study. Because TIMED is a low-altitude satellite, each auroral image only covers ~ 1/3 to ~ 1/2 of the auroral oval.
We use the level-2 data of the auroral energy flux (Q) inferred from the GUVI LBH-long band emission. The derivation of the energy flux, presumably all deposited by electrons, is given in detailed by Zhang and Paxton (2008). To facilitate the analysis, the GUVI data are binned to uniform grids with 1° in latitude and an equal length in longitude. The altitude adjusted corrected geomagnetic (AACGM) coordinate (Baker and Wing 1989) is used for the present analysis.
The auroral energy flux in the prenoon sector covers a wider (~ 10°) latitude than does the postnoon counterpart, and is not isolated from the nightside aurora (not shown). This region of the oval is on the drift path of earthward convecting electrons from the central plasma sheet. Auroral precipitation is dominantly of diffuse type and is closely related to nightside activity. Overall, the prenoon auroral energy flux is slightly larger for positive IMF By for both hemispheres and is larger in SH than in NS regardless of the IMF By polarity. There is evidence that the westward auroral electrojet is more intense for positive than negative IMF By (e.g., Laundal et al. 2016; Friis-Christensen et al. 2017). We do not know if this is also true for diffuse aurora, although it is tempting to speculate that the two are related. It deserves a separate study from the present one.
We have explored the long standing question about effects of IMF By orientation on the morphology of the dayside aurora for the Northern and Southern Hemispheres. It is shown, under similar solar wind driving, the postnoon auroral hot spot, which is centered at 1500 MLT in the Northern Hemisphere (Newell et al. 1996; Liou et al. 1997), also exist in the Southern Hemisphere and at the same local time. It is also found that in the postnoon 1400–1600 MLT sector of the Northern Hemisphere, the peak auroral energy flux is larger (~ 11%) for negative IMF By than for positive IMF By. The postnoon aurora can reach higher latitudes for positive IMF By compared to negative IMF By, resulting in larger energy flux there for positive than negative IMF By. This IMF By polarity dependence for the postnoon aurora is reversed and more pronounced in the Southern Hemisphere—the peak auroral energy flux is larger (~ 23%) for positive than negative IMF which are also present asymmetricy. In the prenoon sector, the auroral response to the IMF By orientation is in general opposite to that of the postnoon aurora but less pronounced. This result suggests an asymmetrical response of the aurora in the dawn and dusk sectors to the orientation of IMF By.
The present result that a clear IMF By effect on the postnoon aurora and a weaker IMF By effect on the prenoon aurora is consistent with the recent finding that upward field-aligned currents are well correlated with auroral precipitation in the dusk sector, but not in the dawn sector (Korth et al. 2014). The present finding is, to a certain extent, also consistent with the reports that negative IMF By favors the occurrence of auroras in the afternoon sector (e.g., Murphree et al. 1981; Vo and Murphree 1995; Trondsen et al. 1999). One must note that these studies are qualitative and therefore cannot address the absolute intensity of the aurora. These authors have associated their findings to the field-aligned currents and ionospheric convection, which also appear to response asymmetrically to the orientation of the IMF By component (e.g., Burch et al. 1985). The present finding of the lack of dependence of the total auroral energy flux deposited in the postnoon sector (1400–1600) on the IMF By orientation is consistent with the finding reported by Liou et al. (1998), who found the total auroral power in the afternoon (1300–1800 MLT) is linearly proportional to the magnitude of IMF By. This can also explain why Yang et al. (2013) did not find a dependency of the afternoon auroral intensity on the orientation of IMF By component. Based on the present and these previous results, the orientation of IMF By does not lead to more energy precipitation in the postnoon sector but affects the precipitation pattern in the ionosphere. In the Northern (Southern) Hemisphere, a negative (positive) By component changes the precipitation pattern such that the postnoon is more focused and brighter. This is the reason why previous studies often found enhanced auroral intensity in the Northern Hemisphere during IMF By < 0.
The topology change of the magnetosphere is an important manifestation of solar wind–magnetosphere coupling, and magnetic field merging/reconnection plays the key role in the coupling. The anti-parallel merging model (Reiff and Burch 1985) predicts two merging sites in opposite side of the noon in each hemisphere for a finite IMF By component. It is generally believed that this north–south asymmetry in the merging sites, when coupled with the solar wind drag, is responsible for the dawn–dusk asymmetry in the ionospheric plasma convection (e.g., Ruohoniemi and Greenwald 1996), field-aligned currents (e.g., Burch et al. 1985; Weimer 2001), and perhaps dayside auroras (e.g., Murphree et al. 1981).
In the dawn sector, however, the present result does not support the convection model, which predicts little influence in the dawn sector because the region-1 field-aligned current is dawnward and is not associated with particle precipitation. While diffuse auroras dominate the dawn sector particle precipitation because nightside electrons drift dawnward, auroral arcs can also appear, thought less frequent and intense, in the dawn sector (e.g., Newell et al. 1996). It is possible that field-aligned currents associated with auroral arcs in the dawn sector also enhanced in response to a stronger convection for IMF By > 0. However, we are not aware of any report about field-aligned current enhancements associated with the orientation of IMF By.
The convection model may explain the expansion of the postnoon aurora to higher latitudes for positive IMF By. When IMF By is positive, the larger dusk cell can expand to high latitudes, as well as its associated field-aligned currents, as shown in Fig. 6a. Electron precipitation associated with the field-aligned currents can produce auroras, probably weak, at higher latitudes. A little change in the prenoon aurora is expected because the field-aligned currents are dawnward. This is not consistent with our observations, which show an opposite response comparing to the postnoon aurora. On the other hand, it has been reported that the center of the Southern polar cap, when fitted with a circle, shifts duskward when IMF By > 0 and dawnward when IMF By < 0 (Holzworth and Meng 1984). This is consistent with our result shown in Fig. 5b, d. Dayside anti-parallel magnetic field merging can occur in the high-latitude afternoon magnetopause for IMF By > 0 and high-latitude prenoon for IMF By > 0. Open magnetic fluxes are pulled into the lobe from dayside, through the noon-midnight meridian, to the nightside of opposite quadrant due to the magnetic tension force. This effect is opposite between the two hemispheres. Therefore, it is expected that the Northern oval moves dawnward for IMF By > 0 and duskward for IMF By < 0, as shown in Fig. 5a, b.
The present result of a hemispheric asymmetry in the dayside aurora may also suggest existence of an interhemispheric current associated with a finite IMF By component, in addition to the large-scale region-1 and region-2 currents systems responsible for steady-state convection. A field-aligned current system associated with the IMF By component has been theoretically considered (e.g., Leontyev and Lyatsky 1974). According to this model, the solar wind motional electric field, after reconnection, polarizes the polar cap (open flux) region with opposite charges, depending on the orientation of IMF By, setting up field-aligned currents. The field-aligned currents can flow in the ionosphere into low latitudes and form a thin layer of interhemispheric field-aligned currents in the close region (Kozlovsky et al. 2003). A schematic drawing of this model is shown in the bottom panels of Fig. 6. As shown in Fig. 6c, a positive IMF By component will induce an interhemispheric field-aligned current out of the Southern Hemisphere, whereas a negative IMF By component will induce an interhemispheric field-aligned current out of the Northern Hemisphere. The IMF By-induced interhemispheric field-aligned current implies a larger (weaker) auroral intensity in the hemisphere where currents are flowing out. Therefore, this model predicts that dayside auroras are more intense in the Northern Hemisphere for IMF By < 0 and in the Southern Hemisphere for IMF By > 0. The present analysis result clearly indicates that the condition is more completed. Since this model applies perhaps only to regions close to the open-closed boundary, Fig. 5a, b indicates that the model correctly predicts the dawn sector but not the dusk sector for both hemispheres.
Finally, the present finding provides an impact in space weather forecasting. With simplicity in mind, current auroral models are parameterized by the Kp index (Hardy et al. 1991) or the solar wind coupling function (Newell et al. 2014). These models are not capable of simulating the IMF By effect as presented here because both parameters are not taking the IMF By orientation into consideration. Second, these models assumed symmetric auroral ovals and were constructed using combined data from both hemispheres. This assumption is clearly not valid as it is clearly shown in the present result. Future work on improvements of these models based on the present result is crucial for accurate aurora forecasting.
We have performed a statistical analysis of the auroral images in FUV acquired by the TIMED/GUVI spectrograph imager under dark conditions. Surprisingly, the so-called 1500 MLT auroral hot spot also exists in Southern Hemisphere and at exactly the same local time. It is also found that the IMF By orientation plays an important role in the dayside auroral morphology. A few salient findings are listed below: (a) The energy flux in the postnoon sector becomes more focused and the peak value is ~ 11% (23%) larger in the Northern (Southern) Hemisphere for negative (positive) than for positive (negative) IMF By, but the total intensity does not change with the IMF By orientation; (b) A weaker response to the IMF By orientation is found for the prenoon aurora, which shows an opposite response to the IMF By orientation comparing to the postnoon aurora; (c) The asymmetric response of dayside auroras to the IMF By orientation is expected to form a north–south asymmetry in the dayside aurora, especially in the postnoon sector. Part of the present finding can be explained by the ionospheric convection flow model.
The GUVI instrument was designed and built by The Aerospace Corporation and the Johns Hopkins University. The Principal Investigator is Andre B. Christensen and the Co-PI is Larry J. Paxton. The GUVI data reported herein are available through the TIMED/GUVI website at http://guvitimed.jhuapl.edu. The 1-min IMF and solar wind data were provided through OMNIWeb by the Space Physics Data Facility (SPDF) (ftp://spdf.gsfc.nasa.gov/pub/data/omni/highresomni/).
All authors worked on the project and wrote the final manuscript. Both authors read and approved the final manuscript.
This study was supported by the NSF (# 1743118) and AFOSR (# 26-0201-51-62) Grants to the Johns Hopkins University Applied Physics Laboratory.
The authors declare that they have no competing interests.
- Paxton LJ, et al (1999) Global ultraviolet imager (GUVI): measuring composition and energy inputs for the NASA thermosphere ionosphere mesosphere energetics and dynamics (TIMED) mission, in optical spectroscopic techniques and instrumentation for atmospheric and space research III. In: Larar AM (ed) Proceedings of SPIE, vol 3756, pp 256–276Google Scholar
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