In the present case study, two distinct auroral brightenings were observed in ground ASIs, as expected: the AIB and the following poleward expansion a few minutes later. This two-stage development is consistent with the classic Akasofu substorm onset (Akasofu 1964) and presumably corresponds to two different physical mechanisms.
In contrast, the AIB, which was observed in the ASIs, was not evident in the GIs, as illustrated in Fig. 11. Consequently, the identified first brightening in the GIs corresponded to the second brightenings in the ASIs (i.e., the poleward expansion). In this section, we discuss these differences between ASIs and GIs, including time delay, causes, implications for the onset definitions, and impacts on the reconnection timing.
Time delay of substorm onsets between ground and satellite images
In the present study, the substorm onset identified by using GIs was delayed from the ASI data by 3 min. This delay corresponds to the time difference between the AIB and the poleward expansion and thus corresponds to the duration of the first stage (Fig. 1b) of the substorm expansion phase in Akasofu (1964) of a few minutes. This Stage 1 often includes auroral rays (Akasofu 1964). We believe that auroral rays and auroral beads are different views of the same auroral structure and that both can be recognized as detailed features of a longitudinally wide brightening (i.e., AIB in Fig. 11).
The duration of the AIB in the ASIs was 2.5 min (Rae et al. 2009), a few minutes (Mende et al. 2009), and 7 min (Motoba et al. 2014) in previous case studies. The duration was 1–2 min on average and extended to 7 min in a statistical study (Nishimura et al. 2016). Thus, large diversity occurs in the identified delays/durations (\(\sim 1{-}7\hbox { min}\)). In the present discussion, we assumed that the time delay is typically a few minutes.
It is currently difficult to comprehensively understand this diversity, although a clue may be that the AIB tends to have a short duration when it intensifies rapidly (Nishimura et al. 2016). Practically, precursor brightenings are often observed prior to the AIB (e.g., Ieda et al. 2016). It is sometimes difficult to determine whether such a brightening is the AIB or a precursor, particularly when it does not decay significantly, leading to subjectivity in the duration of the AIB. Substorm onsets with a delay/duration shorter than the time resolutions of the GIs would appear to be simultaneous between the ASIs and GIs. Even in such cases, the implications of observed onsets are presumably different between the ASIs and GIs.
The delay of GI onsets from ASI onsets has been assumed to be small, at less than \(\sim 1\hbox { min}\) (e.g., Liou 2010), without direct comparison of GIs and ASIs. Pi2s have been classical substorm onset signatures (e.g., Rostoker et al. 1980; Olson 1999; Nosé et al. 2012). GI onsets have been observed \(\sim 1\hbox { min}\) prior to Pi2s (Liou et al. 2000). This correspondence may verify that the delays of GI onsets from ASI onsets are small. However, the present study and Ieda et al. (2016) suggest that major Pi2s are not likely associated with the Akasofu substorm onset, but rather with the poleward expansion later in the ASIs. Thus, the correspondence of GI onsets (i.e., poleward expansion) to Pi2s does not necessarily imply that the delays of GI onsets from ASI onsets are small. Rather, it suggests that the substorm onsets in the GIs are delayed with respect to the AIB in ASIs by more than that expected, depending on the duration of the AIB.
Causes of differences between ground and satellite images
Poleward expansion was observed in both ASIs and GIs. This sudden change appeared to be even more evident in the GIs (Figs. 6b and 7b) than in the ASIs (Figs. 6a and 7a), indicating that the practical sensitivity of the GIs is sufficient to identify poleward expansion. In contrast, the AIB was not evident in the GIs, indicating that the sensitivity of GIs is considerably less than that of ASIs for identifying the AIB.
These results suggest that the different responses between ASIs and GIs may depend on the latitudinal thickness of the auroras. Our interpretation is that the brightness of the aurora is underemphasized when the target is thinner than the spatial resolution of images. This underemphasis is attributed to the averaging of an area that includes both the thin aurora and the adjacent dark region. The AIB is less evident in GIs, presumably because it is thin in terms of the latitude range, particularly at the beginning, compared with the spatial resolution of GIs. Thus, its brightness would be reduced significantly by area averaging. In contrast, the poleward expansion includes a thickening of the bright aurora; thus, its brightness would be reduced at the beginning but would not be reduced after the expansion has reached the spatial resolution of GIs. That is, the increase in brightness would be overemphasized in GIs when it begins to detect poleward expansion (i.e., auroral breakup).
Another possibility is that these different responses in ASIs and GIs may be attributed to the difference in wavelengths used to observe the auroras. The difference in wavelength did not result in significant differences in the brightness of the poleward expansion. However, it may contribute to difference in the brightness of the AIB. Both satellite (170 nm) and ground (557.7 nm) images are expected to be sensitive to precipitating electrons in the keV range. Thus, the difference in wavelength likely did not contribute significantly to the difference in observed auroras if the onset was dominated by keV-range electrons. However, precipitating electrons may belong to other energy ranges for the AIB. In such cases, the difference in wavelength may contribute to the different responses.
The AIB was not evident in the GIs in the present case; in other cases, the AIB may be sometimes visible in GIs depending on the conditions of auroras and cameras. However, the wide brightening is not explicitly included in identifications of substorm onset in GIs (e.g., Frey et al. 2004; Liou 2010), although it is not explicitly excluded. Thus, the AIB has not been typically recognized in GIs thus far. The AIB would be difficult to recognize as a substorm onset (i.e., sudden brightening) in GIs, not only because its brightness is underemphasized, but also because the increase in brightness of the following poleward expansion is overemphasized. With these assumptions, it may be sometimes possible to recognize a weak brightening in GIs as belonging to the AIB a few minutes prior to the major brightening (i.e., poleward expansion).
Clarifications of substorm onset definitions
We have inferred that the traditionally identified onset brightening in satellite GIs does not necessarily correspond to the Akasofu substorm onset. Instead, it tends to represent the poleward expansion that follows a few minutes later (Fig. 11). Below, we discuss the reason why this interpretation has not been widely recognized.
Confusion regarding two different localized brightenings
Substorm onsets in GIs are traditionally identified by a localized brightening, which is labeled as auroral breakup (e.g., Frey et al. 2004; Liou 2010). Note that the two-stage development of the Akasofu model has not been required in these identifications, presumably because of the limited sensitivity of GIs. In contrast, this localized brightening in GIs is sometimes (e.g., Frank et al. 2001b; Morioka et al. 2014) specifically labeled as the (Akasofu) IB instead.
This confusion arises likely because it is not often recognized that the AIB (Akasofu 1964) is elongated along longitudes instead when considered on a timescale of a few minutes. This wide AIB may appear as localized (\(\ll 1\hbox { MLT}\hbox { hour}\)) at the very beginning (\(\sim 10\hbox { s}\)) in the ASIs (e.g., Liang et al. 2008) (Fig. 11). However, this weak aurora at the very beginning can be marginally recognized only on detailed inspection of ASIs; thus, it is expected to be barely detectable by GIs owing to the limited sensitivity and time resolution.
Moreover, such localized brightenings expand quickly in longitude, and the resultant wide aurora, sometimes including auroral beads, should be more evident than localized auroras. It is unlikely that the localized aurora at the very beginning was observed without observing the following brighter wide aurora. Thus, the observed localized brightening in GIs is unlikely to correspond to the localized brightening at the very beginning of the AIB in ASIs, at least in most cases.
As discussed above, the localized first brightenings in ASIs and GIs are not likely to represent the same phenomenon. This difference has not been often appreciated, likely also because both brightenings are referred to as “localized.” The first brightening in the GIs appears to be localized in wide-area images such as the 2128:07 UT panel of Fig. 3, but the same brightening does not appears to be localized in expanded images such as that in Figs. 6b and 5. Thus, the term “localized” has different implications between ASIs and GIs (Fig. 11) depending on the size of the displayed area.
Confusion regarding expansion onset and expansion phase onset
As discussed above, localized brightening in GIs is sometimes confused as corresponding to the Akasofu substorm onset. The same confusion arises likely because “expansion phase onset” sounds like the start of poleward expansion. One such example is a statement of (McPherron 2016): “The instant at which the aurora begins to expand poleward is called the onset of the expansion phase of the auroral substorm (Akasofu 1964).” This recognition is inconsistent with Akasofu (1964), as explained below.
A substorm is traditionally divided into three phases: the growth phase, the expansion phase, and the recovery phase. Substorm onsets refer to the beginning of the expansion phase (e.g., Baumjohann and Treumann 2012). The term “substorm onset” may be confused with the start of the growth phase and is often explicitly referred to as the “substorm expansion phase onset,” which is the beginning of the expansion phase, as this term itself defines.
The expansion phase is defined in Akasofu (1964) to begin with Stage 1 (AIB, i.e., without poleward expansion), followed by Stage 2 (poleward expansion) a few minutes later (Fig. 1). Thus, confusingly, there is no poleward expansion at the beginning of the expansion “phase” onset in the Akasofu substorm model. That is, “the instant at which the aurora begins to expand poleward” does not correspond to the expansion “phase” onset by definition.
Initial brightening or poleward expansion as a substorm onset
The two-stage development in the original definition of substorm onset has not been emphasized in later studies. For example, Rostoker et al. (1980) summarized various signatures to identify substorm onsets to include auroral arc brightenings, negative bays, positive bays, and Pi2s. Meng and Liou (2004) identified substorm onset as an auroral breakup, which they defined as a sudden brightening followed by poleward expansion. Such studies did not discuss these signatures in the context of the two-stage development; rather, they implicitly assumed only one stage.
In contrast, different stages have been used to define substorm onsets in recent studies. The AIB (i.e., the original definition, Stage 1) is sometimes adopted to identify substorm onsets (e.g., Donovan et al. 2008). Poleward expansion (i.e., Stage 2) is instead adopted with (e.g., Mende et al. 2009) or without (e.g., McPherron 2016) the recognition that this and the original definition differ. Substorm onsets in GIs are usually identified by the sudden brightening (e.g., Frey et al. 2004; Liou 2010). In contrast, Morioka et al. (2014) recognized in GIs that the sudden brightening is followed by another brightening a few minutes later; they identified the substorm expansion phase onset by this second brightening in GIs.
As summarized above, the definition of a substorm onset (i.e., substorm expansion phase onset) is currently diverging and is sometimes confused. To avoid such confusion, individual studies that include discussions within a few minutes of accuracy are recommended to state the definition of substorm onsets explicitly in the context of two-stage development. Two major possible definitions, AIB and poleward expansion, are discussed below.
If the substorm onset is defined as the first signature, it is likely to correspond to AIB, which is the original definition of onsets. Practically, this onset can be regularly monitored only by using ASIs. It may include auroral rays or beads and is often too evident to ignore before the beginning of poleward expansion. The AIB may be a manifestation of the triggering process of substorms, such as near-earth instabilities or the initial stage of tail reconnection. Even the AIB may play an active role in triggering substorms, for example, by feedback processes with the enhancement of ionospheric conductance and current. However, it may also be possible that the AIB is not directly associated with substorm onsets and occurs under background conditions favorable for the occurrence of substorm onsets.
In contrast, if the substorm onset is defined as the beginning of an explosive release of energy from the tail to the polar ionosphere, it is likely to correspond to poleward expansion. The poleward expansion presumably maps to dipolarization in the tail (e.g., Chu et al. 2015), thus manifesting the explosive energy release. Because the dipolarization is a drastic change in the magnetic field lines, it would cause major magnetic oscillations (i.e., major Pi2s). This onset can be identified by using various data sets such as GIs and geomagnetic fields in addition to ASIs and is thus useful at least as a working definition. However, it should be remembered that poleward expansion is not the original definition (Akasofu 1964) to time the substorm onsets.
Impacts on past tail reconnection timing
Reconnection-associated fast plasma flows are often observed in the magnetotail near the time of a substorm onset identified by using Pi2s or GIs (Hones et al. 1984; Moldwin and Hughes 1993; Slavin et al. 2002; Ieda et al. 2008). These fast flows have occasionally been further identified a few minutes prior to the substorm onset (Nagai et al. 1998; Ohtani et al. 1999; Baker et al. 2002; Kepko et al. 2004; Miyashita et al. 2009).
However, such conclusions depend on the definition of substorm onset. Whether the identified substorm onset corresponds to the AIB or poleward expansion in ASIs has not been specified in these previous studies. In the present study, the onsets in Pi2s and GIs corresponded to poleward expansion rather than the Akasofu substorm onset. This result suggests that unless the longitudinally wide AIB was explicitly considered, the substorm onsets identified in past studies did not correspond to the Akasofu substorm onset but rather to poleward expansion.
Fast flows have always been initiated within a few minutes of the isolated auroral breakup in GIs (i.e., poleward expansion) if the satellite was located near the onset MLT (Ieda et al. 2008). However, unobserved AIB may have occurred prior to the auroral breakup (i.e., poleward expansion). Thus, these fast flows may be delayed from the possible AIB, as was reported in a case study by Ieda et al. (2016). In summary, no evidence exists for reconnection-associated fast flows prior to the Akasofu substorm onset. Therefore, the developed reconnection does not likely trigger the Akasofu substorm onset.
Reconnection-associated fast flows may be associated with auroral streamers. Some brightenings (e.g., 2126:17 UT panel) occurred near 73 MLAT near the onset MLT sector in Fig. 3a and b. Interestingly, an auroral streamer was formed at 72 MLAT near the onset MLT simultaneously with the breakup (2128:07 UT panel). This simultaneous occurrence may be a coincidence, or it may suggest that the auroral breakup (i.e., poleward expansion) and tail reconnection occur simultaneously.