The ionospheric ions that flow out into the magnetosphere include the polar wind, upwelling ions from the cusp, polar cap, and ion beams accelerated in the aurora by the electric field parallel to the magnetic field direction. These escaping ions have been observed by experiments from radars on the ground and on numerous satellites including DE1, Polar, Geotail and Cluster . The polar cap ions include the polar wind and cusp origin O+ . Cold polar cap ions sometimes consist of mainly H+ ions and can dominate the lobe outflow fluxes . Ions flowing out with transpolar arcs (TPAs, also known as theta auroras) are field-aligned and observed when the interplanetary magnetic field (IMF) is northward . Case events and statistical studies have shown the ion beams flowing out of the polar cap are similar to auroral beams accelerated by field-aligned potentials . However, cold O+ beams may form from velocity dispersion, and the beams can look similar to those accelerated by an electric field .

The energetic ions flowing out of the auroral oval into the magnetosphere occur mainly during auroral substorms. studied statistically the ion composition in the plasma sheet as a function of substorm activity. In particular, they studied whether substorms occurring during magnetic storms are different from substorms in non-storm times. The role of O+ ions in substorm dynamics is still unresolved. Observations on the one hand have shown that O+ ions play a significant role in substorm dynamics , while others indicate the O+ ions have no effects .

POLAR Tide experiment together with Ultraviolet Imager auroral images have established that the outflowing O+ ions are dependent on the substorm intensity, with outflowing fluxes increasing as the substorm intensity increases . We have examined if the substorms are the only source of auroral ions flowing out of the auroral oval. In this article, we show evidence that auroral ions do not only escape during substorms but also during periods without substorms. Periods without substorms include “quiet” arcs, TPAs and pseudo-breakup auroras. This article will focus only on observations during quiet arcs and pseudo-breakup auroras.

A pseudo-breakup aurora is different from a regular substorm breakup. Pseudo-breakup auroras involve activation of a small area of an arc that does not expand globally . In pseudo-breakup auroras, a section of an arc brightens “momentarily” and then fades. This process can occur many times, but this behavior is different from a regular substorm breakup, which includes the growth, expansion and recovery phases . While a distinction is made between substorms and pseudo-breakups, numerous studies from ground and space have shown that pseudo-breakup auroras include all of the same features associated with auroral substorms, except the intensities are weaker. For example, Pi2 magnetic oscillations, which signify the onset of a substorm, are observed with the brightening of pseudo-breakup auroras but the amplitudes are smaller . In the geomagnetic tail, the magnetic field undergoes “limited” dipolarization, accelerating electrons to several hundred kiloelectron volts and ions to a few megaelectron volts but the fluxes are lower . This article will add another feature to the list of pseudo-breakup auroras: acceleration of H+, O+ and He+ ions upward out of the auroral oval ionosphere, in a nearly identical way to ions accelerated along the magnetic field in regular substorms.

The outflowing ions during periods without substorms have been measured by the ion composition experiment known as Composition Distribution Function (CODIF) on Cluster . Periods of substorms and those without substorms are determined using auroral images obtained by the Wideband Imaging Camera (WIC) on the IMAGE spacecraft aided by the list of substorm onsets identified by , Auroral Electrojet (AE) indices (from the World Data Center for Geomagnetism, Kyoto AE index service) and ground-based all-sky camera records of the aurora obtained by MIRACLE (Magnetometers – Ionospheric Radars – All-sky Cameras Large Experiment) in northern Scandinavia.

A random survey of ion composition data from Cluster for 30 different days (Table 1) has revealed that most of the outflowing ions occurring during periods without substorms are associated with the auroral oval whose activities included the quiet arcs and pseudo-breakup auroras. The measured energies of the escaping O+ ions are typically ∼20–40 eV, but after correcting for the spacecraft potential, the ion beams have energies of ∼40 to 80 eV. These potentials are lower than the typical potentials associated with substorms which are >100 eV to several kiloelectron volts .

Cluster measured the ions flowing out at heights of ∼2 RE to >10 RE. The fluxes of outflowing ions are estimated to be >103–105 cm-2s. Assuming the source of these ions is the auroral oval area sampled by the WIC, the estimated flow rate of ions during periods without substorms is ∼1020 s-1. Noting however that quiet arcs and pseudo-breakup auroras can persist for hours, our results show that the non-substorm contribution of the ions we measure can be very significant for the plasma sheet. We have mapped the footprint of Cluster 3 using Tsyganenko 89 and 96 models to show that the spacecraft are located in the auroral oval. By comparing our results with those in and with the discussion in , one may find that not much heating and centrifugal acceleration is expected along these outflow paths in the near-Earth lobes to the plasma sheet. The auroral ions have similar and different features from the ions in the polar wind, upwelling ions (cleft ion fountain) and polar cap . In this first report, we will show three examples from periods with different geomagnetic disturbance levels under which the ion beams were observed flowing out of the ionosphere. Ions from both day and night sides flow out but we do not distinguish between them here, nor will the observations be compared to acceleration models .

The list of events shown in Table 1 includes the time intervals of the events, the position of Cluster covered during the interval (first number in X, Y, Z represents start position and the number that follows in parenthesis is the end position), and the time of the last substorm that occurred just before the observations started, based on available WIC images and AE data. As can be seen, the observation intervals did not include any substorm activity and they generally started at least several hours after a substorm had occurred. Three examples from this list are presented for further discussion: 31 October 2001, 19 March 2001 and 19 November 2001 (Fig. 1).

In Fig. 1a, b and c, panels 1, 3 and 5 show the differential number flux spectrograms of H+, He+ and O+ ions with energies of ∼25 eV–40 keV per charge, panels 2, 4 and 6, the density of these ions (statistically significant number counts measured by the instrument corresponds to densities ≥0.01 cm-3). The overlapping IMF components and AE indices for these times are shown in Fig. 2a, b and c. The IMF Bz component on 31 October was predominantly negative but small; on 19 March 2001 it was also negative from 04:00 to 06:00 UT after which it became positive (data missing from 07:30 to 09:00 UT); and on 19 November 2001 it fluctuated between positive and negative values. The IMF behavior here indicates that the events we discuss are different from periods when the polar cap beams flow out .

The keograms covering the same period constructed from the individual WIC Far Ultraviolet Imager images are shown in Fig. 3a, b and c. In each keogram, the top panel shows the auroral intensity as a function of MLat, the second panel as a function of magnetic local time (MLT), and the third panel the integrated photon flux averaged over 18:00–06:00 MLT and 50–80∘ MLat. The solid black lines are footprints of Cluster 3 in the Southern Hemisphere using T89 and T96 models (the two models essentially gave the same results) for 31 October, 19 March and 19 November 2001 (caveat: the keograms come from auroras in the Northern Hemisphere). The footprints of Cluster on 31 October were at MLat > 80∘ so are not shown. Figure 4a, b and c shows examples of individual auroral images to illustrate the activities in the auroral oval and pseudo-breakup auroras. Examples of the velocity space distribution of O+ ions for the three days are shown in Fig. 5. Figure 6 shows the velocity distributions of the three ion species (H+, He+ and O+), all from 19 November 2001.

31 October 2001: On this day, no significant H+ and He+ were measured. Only O+ ions were measured with very low density ∼0.01 cm-3 after ∼05:40 UT. The keogram (Fig. 3a) shows no significant auroral fluxes until about 06:30 UT. The average fluxes corresponded to ∼350 rayleighs (R) at 04:00 UT which increased to 500 R at 06:30 UT. The AE indices between 04:00 and 06:30 UT were nearly zero (Fig. 2a), corroborating the keogram plot. The WIC has a spatial resolution of ∼50–70 km, hence auroral arcs are not spatially resolved. Thus, the WIC observations have been augmented using all-sky camera records from MIRACLE. For this day, we find quiet auroral arcs, measured at 557.7 nm, present until ∼03:00 UT when the observations ended. There were no substorms recorded (not shown).

Individual auroral images (Fig. 4a) show that the small auroral intensity increases observed at 06:30 UT and 07:00 UT occurred near midnight meridian at ∼70∘ MLat and were due to brightening of the aurora and pseudo-breakups, which increased the AL indices to a peak value of ∼50 nT. The increased auroral fluxes at ∼07:40 UT are due to a more intense pseudo-breakup aurora which increased the AL index to ∼100 nT (Fig. 3a). However, no expansion occurred and hence this intensification was considered due to pseudo-breakup activity.

19 March 2001: Unlike 31 October 2001, significant fluxes were measured in all of the ion species, H+, He+ and O+, between 05:00 and 07:00 UT (Fig. 1b). The density of H+ started out at ∼0.1 cm-3 at 05:00 UT which decreased to 0.01 cm-3 at 06:30 UT. The He+ fluxes were weak with densities of <0.01 cm-3, which however occasionally reached ∼0.01 cm-3. The O+ fluxes were the highest with the density around 0.1 cm-3 from 05:00 to 07:15 UT.

The geomagnetic activity was also slightly higher than the previous example. The keogram for this day (Fig. 3b) indicates that auroral activity between 04:00 and 08:00 occurred from 62.5 to 70∘ MLat (top panel) and the activity was in the evening and morning sectors (middle panel). The integrated photon fluxes (bottom panel) corresponded to ∼550–1000 R between 04:00 and 09:00 UT. Note that Cluster footprints initially traversed the region with small auroral fluxes (∼07:00–09:30 UT). Cluster then traversed regions with slightly enhanced intensity, observed around 09:40 UT, resulting in a 200 nT AU increase (Fig. 2b). These auroral fluxes are associated with pseudo-breakup aurora, which included isolated activities at the northern and southern boundaries of the auroral oval, but no substorm expansion occurred (Fig. 4b). According to , a substorm on 19 March 2001 occurred at 09:25 UT and the prior substorm was on 18 March 2001 at 21:27 UT (not shown). A substorm growth phase was observed beginning around 09:15 UT (see the keogram) that led to substorm onset at 09:25 UT (Fig. 2b, 3b, 4b).

19 November 2001: Figure 1c shows the differential number flux spectrograms of the species H+, He+ and O+ (06:00–10:00 UT). The H+ density was ∼0.08 cm-3, O+ density 0.32 cm-3 and He+ density <0.01 cm-3. This and the last example show that the O+ ions dominated H+ ions, though not all cases show this tendency. We calculated and averaged the ratios O+/H+ for each of the 24 events we examined that had simultaneous WIC images and Cluster ion data. This resulted in half of them having an average ratio O+/H+ of ∼2.46±1.57 and the other half having a ratio of ∼0.08±0.04 (not shown). The ratios of O+/He+ were all >1 with the average equaling ∼25, except for two cases. Our observations are different from previous observations (for example FAST) where H+ ions generally dominated . However, FAST observations came from disturbed substorm times, and the results are from statistical studies. Note that in rare cases, He+ ions dominate H+ and O+ ions .

Of the three examples, this day (19 November 2001) was the most disturbed with the average integrated photon fluxes starting out around 1.25 kR until ∼10:30 UT, when a substorm onset occurred (also identified by ). The keogram shows (Fig. 3c) moderate auroral activity during 06:00–10:00 UT with pseudo-breakup activities indicated by the individual images (Fig. 4c; comparison of the first and fourth images shows a spot at 21 MLT disappearing and reappearing again). The AE for ∼10:30 UT substorm peaked around 500 nT (Fig. 2c), and the individual auroral images show an expansion of the aurora (not shown). For this day, the footprints of Cluster were mapped to the latter part of the auroral activity, hence we have no direct information on the regions of the pseudo-breakup activity.

Distribution function: Figure 5 shows examples of the velocity space distributions of O+ observed for the three days discussed above. Ion beams are observed by all three Cluster spacecraft (SC1, 3, and 4) but we only show data from Cluster 1. These are 2-D cuts of the 3-D distributions presented in the spacecraft frame where the coordinates are relative to directions parallel (Vpar) and perpendicular (Vperp) to the magnetic field. The scales of the x and y axes are ±50 kms-1. On these three days, Cluster was traversing the Southern Hemisphere, and the positive Vpar corresponds to ions flowing parallel to the magnetic field direction out of the ionosphere. All of the ions are field-aligned beams occupying a very small region of the velocity space (one or two pitch-angle bins closest to the direction of B), and the measured O+ beams have a velocity of ∼20 kms-1 (for spacecraft potential correction, see below).

While the O+ beams can form from velocity filter effects , we will interpret our observations in terms of particle acceleration by parallel potential drops because the distributions we measured were very much confined to the parallel direction . In this case, the energy per charge of an ion that has gone through a potential drop Δϕ measured by CODIF is mv2/2q=(Wth/q)+Δϕ, where Wth is the thermal energy. If the initial thermal energy of the ambient ionospheric plasma (<1 eV) is assumed to be much less than the potential drop (Wth≪qΔϕ), then the shift of the peak of the ion beam will correspond to the energy gained by going through the potential drop. The observed beam velocities of 20 kms-1 indicate the O+ ions have undergone a potential drop of ∼20–40 V Δϕ=mv2/2q=16×1.6×10-27×(20×103)2/2×1.6×10-19=32 V along the magnetic field. However, the spacecraft was charged positively to ∼10–40 V, indicating the actual beam energy is ∼50–80 V.

Figure 6 shows the velocity space distributions of the ions measured on 19 November 2001. The measured velocity of the H+ beam is ∼100 kms-1, of He+ is ∼50 kms-1 and of O+ ∼20–30 kms-1. The measured ratios of beam velocities Vb of O+ relative to H+ is ∼4 and of He+ ∼2. These values are exactly the same as the theoretical ratios of the beam velocities of O+ and He+ relative to the H+ if all of the ions had gone through the same potential drop: VH+=2VHe+ and VH+=4VO+. Thus, our observations indicate that the three ion species originated from nearly the same height and went through ∼50–80 V of potential drop. However, note that O+ ions showed velocities extending to higher values, indicating that O+ ions went through a larger potential range.

For a Maxwellian distribution, the width of the distribution corresponds to the temperature of the beams, hence our observations show that the temperature of O+ is larger than H+ and He+. The temperature is mass dependent. The estimated temperatures (T) of H+, He+ and O+ ions (in units of kT where k is the Boltzmann constant) using the distribution function are ∼50, 75 and 200 eV, respectively. The results indicate that ions are not only accelerated by the potential, but they are also heated. Mass-dependent heating has been observed previously , but the details of how such a heating mechanism works still remains unknown. Note that for the O+ we observed, there are counts in channels in the adjacent bins relative to the magnetic field direction and the temperature calculation includes their contribution. These particles are probably pitch-angle scattered particles of the original beam along the field. If only the field-aligned portion is included, the temperature will be reduced to <100 eV. Note also that because of mirror force, the temperature perpendicular to the magnetic field may indicate some heating (see also , for Cluster studies of ion heating throughout the polar cap magnetosphere).

This paper has shown that low-energy ionospheric O+ ions flow out during periods without substorms. The O+ ions are originating from the auroral oval populated by quiet arcs and pseudo-breakup auroras. The ion beams are observed by all Cluster spacecraft although the details are different, indicating the structures are smaller than the SC separations (a few hundred kilometers). Preliminary results of test particle simulation using Tsyganenko model with a Weimer electric field show these ions end up in the lobe and plasma sheet (not shown). However, the results depend on the convective field, which is not measured, and require further studies. Our observations are consistent with previous results that indicate that O+ ions are expected to end up in the plasma sheet .

A qualitative picture that is emerging from these preliminary observations is that the field-aligned O+ ions are accelerated along the magnetic field direction by a potential drop very similar to ions accelerated during substorms . The observed values of the streaming velocity ratios of H+ / O+ and H+ / He+ support the field-aligned acceleration interpretation. However, as noted earlier, a competing mechanism for producing field-aligned beams is velocity filter effects, which is not excluded . The observed potential for periods without substorms is a few tens of electron volts. The escaping energies and fluxes are a few orders of magnitude smaller than in substorms .

The auroral arcs are not resolved by the WIC, whose spatial resolution is ∼50–70 km. Thus, it is not known if the source of the low-energy ions includes only the quiet auroral arcs and pseudo-breakup arcs or if it also includes the larger auroral oval. There is freedom about what size we choose for the source area. If we include the area of 18–06 MLT and 50–80 MLat as in the keogram, the size is about 2.8×1017 cm2. If we look at a smaller region near midnight, say 21–03 MLT and 65–75 MLat, the area is only 3.8×1016 cm2, about an order of magnitude smaller. Based on these numbers the escaping fluxes correspond to a flow rate of 1019–1021 ions s-1. This number is less than the cold ions escaping the polar cap, 1026 ions s-1 . However, considering that the quiet auroral oval can persist for hours, the number of ions escaping here (>1024 ions) can be higher than that predicted by and comparable to the number of energetic ions escaping during substorms .

This article has focused only on the auroral oval that included quiet arcs and pseudo-breakup auroras. However, auroras during periods without substorms also include TPAs, observed during quiet solar wind conditions when IMF Bz is northward. Superficial comparison of TPAs reported by with overlapping FAST ion composition data shows O+ ions were flowing out in some of the TPA events. The electrons of the TPAs have energy spectra similar to the electrons in the plasma sheet , suggesting that TPAs are connected to the plasma sheet. However, what causes plasma sheet electrons to appear in TPAs in the polar cap region is not precisely understood. Observations of TPAs have recently been reviewed in () and references therein).

A fundamental question that still remains unanswered is why pseudo-breakup auroras do not expand. Substorm onsets require dissipation of energy stored in the geomagnetic tail into the ionosphere, and the ionosphere plays an important role . This has suggested that the reason for the non-expansion is that the coupling between the magnetosphere and ionosphere is not adequate and inhibits current flow. But at this juncture, the details remain unclear. Future investigations of non-substorm auroras together with observations of O+ ions escaping the ionosphere and radar measurements of ionospheric conductivity could lead to a better understanding of the coupling of the magnetosphere and ionosphere and the role the ionosphere plays in substorm onset mechanisms.