Uae Radars Network

[Anthony Small]

Therehave been several previous papers that review the entire SuperDARN network (e.g., Greenwald et al. 1995; Chisham et al. 2007) but none have focused exclusively on mid-latitude studies. The purpose of this paper is to review the accomplishments made with the mid-latitude SuperDARN radars. It is hoped this will enhance coordination between the SuperDARN groups in different countries and will help make their accomplishments known to scientists in other research fields. In addition, by looking back on the scientific achievements, one can also look forward to the future. This review concludes with a discussion of several aspects of the future directions of the mid-latitude SuperDARN network.

SuperDARN radars observe scatter from a variety of sources, including ionospheric irregularities, the ground/sea surface, meteor ionization trails, and possibly ice crystals in the mesosphere, sometimes referred to as polar mesosphere summer echoes (PMSE). Thus, these radars are very versatile in terms of the science which they can address. Here, a very brief discussion of the basics of the types of scatter, modes of HF propagation, and operating modes is provided, recognizing that the first two of these will be covered in more detail in terms of the mid-latitude radar observations in later sections.

The signal can take multiple hops to even farther distances, such that the radar can receive scatter from the ionosphere not just directly but from beyond the first location of ground scatter. Moreover, subsequent refraction to downwards propagation can generate bands of ground scatter from more distant ranges.

It is important to note that the latitude of a radar does not specifically determine that it will always measure scatter from a specific region for two reasons. One is simply that the equatorward edge of the auroral oval is not located at the same latitude for all LTs, being at higher latitudes at noon than at midnight (Fig. 5). Furthermore, the level of magnetic activity determines the relative location of a radar with respect to auroral boundaries, and hence, the type of scatter that is received. As magnetic activity increases and the polar cap area increases, the auroral oval expands equatorward while under extremely quiet conditions the oval moves to higher latitudes as the polar cap contracts (e.g., Milan et al. 2003).

Schematic plots showing the a Northern and b Southern Hemisphere SuperDARN radar locations with respect to the auroral oval. The auroral oval (yellow) for moderately disturbed conditions, as quantified by Holzworth and Meng (1975), is plotted as a function of AACGM coordinates with the SuperDARN radar locations identified in green type and green closed circles (polar cap latitudes), blue type and blue closed circles (auroral latitudes), and black type and red closed circles (mid-latitude radars)

Figure 7 shows a schematic illustration of natural phenomena which can be studied by SuperDARN radars. It can be seen that SuperDARN observes a wide variety of phenomena, ranging from polar to mid-latitudes, and from the magnetosphere/ionosphere to the thermosphere/upper mesosphere. Some topics have found new prominence in recent years as a direct result of the development of the mid-latitude SuperDARN radars. Specific examples include sub-auroral and mid-latitude plasma flows, solar flare effects on the ionosphere, and earthquake-triggered ionospheric disturbances.

During periods of enhanced geomagnetic activity triggered by a sustained southward interplanetary magnetic field (IMF), the auroral electric fields associated with magnetospheric convection are known to expand equatorward into the mid-latitude ionosphere (MLAT

Fiori et al. (2010), using the Spherical Cap Harmonic Analysis (SCHA) technique, showed that convection can be increased by moving the lower latitude limit, but caution should be taken with the amount of data going into each pattern. Cousins and Shepherd (2010) demonstrated that the solutions to the statistical patterns were relatively insensitive to the lower latitude boundary.

Ebihara et al. (2008) presented the first mid-latitude SuperDARN observations of the over-shielding electric field using HOK. They examined two reverse flow periods during a moderate geomagnetic storm, the first of which was attributed to over-shielding associated with a northward IMF turning while the second occurred during southward IMF conditions and could not be replicated in the ring current simulation. The second one is probably associated with a substorm. When a substorm occurs, over-shielding is shown to appear at low- and mid-latitudes without northward turning of IMF by global MHD simulation (Ebihara et al. 2014). A later study by Kikuchi et al. (2010) also examined over-shielding signatures during the same geomagnetic storm, although in the context of equatorial DP2 fluctuations were attributed to alternating eastward and westward electrojets in the equatorial ionosphere. Using mid-latitude SuperDARN contributions to the instantaneous global convection patterns, they suggested the dayside reverse flow vortices observed equatorward of the larger two-cell convection correspond to the region 2 FACs responsible for over-shielding at the equator. The study of over-/under-shielding phenomena with mid-latitude SuperDARN radars remains an under-utilized capability and an area for future studies.

To summarize, storm-time convection electric fields corresponding to a variety of geophysical drivers are observed by the mid-latitude SuperDARN radars. Lyons et al. (2016) presented a synthesis of ground- and space-based observations characterizing these fields during the 17 March 2013 geomagnetic storm. They identified an inter-relationship between the expansion of the auroral oval, penetration electric fields, auroral stream activity, and SAPS, which is discussed in the next section.

The question of the nomenclature is ultimately related to the question of physical origins and driving mechanisms. The radial charge separation and the associated polarization electric field in the magnetosphere are widely accepted to be one of the two main drivers, with the other one being positive feedback between the magnetospheric electric field and ionospheric conductance (e.g., Wolf et al. 2007). In this feedback model, the magnetosphere-ionosphere (MI) system is assumed to act as a current generator, with the total current being conserved. In this case, the initial polarization electric field drives ion convection in the ionospheric F region, which increases heating and recombination rates, depleting ionospheric densities, and further strengthening SAPS electric fields (Anderson et al. 1993). Despite a general consensus on the importance of these two processes for SAPS formation and evolution, there appears to be a growing realization that these do not explain some characteristics of narrow SAID (Mishin and Puhl-Quinn 2007; Puhl-Quinn et al. 2007). Similarly, the discovery of highly dynamic and localized plasma flows within SAID/SAPS that are often referred to as the SAPS wave structure or SAPSWS (Mishin et al. 2003; Mishin and Burke 2005) has challenged the view of SAPS as a generally uniform flow region with possibly one or more narrow SAID-like flow channels (Erickson et al. 2002; Mishin et al. 2003; Foster et al. 2004; Mishin and Burke 2005).

In the last decade, SuperDARN has provided numerous contributions to SAPS research that can be divided into the following two groups, roughly corresponding to spatial and temporal features of SAPS. The first exploits the advantage of global coverage of the sub-auroral and auroral ionosphere allowing SuperDARN investigations to improve knowledge of global characteristics and external control of SAPS and, through that, achieve a better understanding of the relative importance of global/external factors versus other drivers. The second exploits the advantage of continuous coverage allowing SuperDARN investigations to improve knowledge of temporal dynamics of SAPS including the importance of the MI feedback mechanism. Studies that address these two categories of issues are respectively reviewed in the following subsections.

Study of the large-scale structure of SAPS has been greatly advanced by the significantly expanded coverage of mid-latitude SuperDARN. In coordination with the original high-latitude radars, the network of mid-latitude SuperDARN radars is unrivaled in its ability to address the spatial characteristics of SAPS on global scales.

The longitudinal structure of SAPS in the sub-auroral and mid-latitude region has been investigated (Oksavik et al. 2006; Koustov et al. 2006; Kataoka et al. 2007; Clausen et al. 2012). In their study, Oksavik et al. (2006) examined a SAPS flow channel equatorward of 60 MLAT that was observed for several hours by WAL. It was revealed that a fast westward flow appeared in the pre-midnight sector, while an eastward flow was co-located on the higher latitude side of the fast westward flow in the post-midnight sector, forming a flow reversal as seen in Fig. 11 (Oksavik et al. 2006). Kataoka et al. (2007) examined a similar flow reversal with HOK and confirmed that the flow reversal is also present in the post-midnight sector and is enhanced during a magnetic storm.

Top/left: spatial distribution of GPS TEC with precipitating electron flux along a pass of NOAA/POES satellite. Bottom/left: 2-D map of LOS velocities observed by mid-latitude SuperDARN radars over North America at 0840 UT on 9 April 2011. Right: Vector representation of the average large-scale SAPS flow direction and the inferred SAPS speed identified by the three radar pairs. Time runs along the y axis, increasing toward the bottom. Adapted from Figs. 5 and 7 of Clausen et al. (2012)

Since the deployment of TIG in 1999 and WAL in 2005 in the Southern and Northern Hemispheres, respectively, continuous observations at sub-auroral latitudes have been carried out, enabling the correlation of SAPS characteristics with solar wind and geomagnetic conditions to be examined. Several studies using SuperDARN data have identified SAPS characteristics which are basically consistent with those obtained with the Millstone Hill incoherent scatter radar (ISR) (Foster and Vo 2002; Erickson et al. 2011), namely, that SAPS tend to form more often, with faster flow speeds, and at lower latitudes with increasing geomagnetic activity level (Parkinson et al. 2005, 2006; Kataoka et al. 2009; Grocott et al. 2011; Kunduri et al. 2012; Nagano et al. 2015; Kunduri et al. 2017). These correlation characteristics strongly suggest that SAPS are closely controlled by solar wind conditions as well as by the ring current. A further examination by Grocott et al. (2011) showed that the latitudinal location of SAID varies on similar time scales to those of the interplanetary magnetic field and auroral activity, while variations in its flow speed are more closely related to ring current dynamics. These results are consistent with the idea that the poleward electric field of SAPS/SAID is caused by the shielding effect of the ring current coupled with the ionosphere through the Region 2 FAC system (Southwood and Wolf 1978).

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