Theoretical considerations , observations e.g. as well as simulations e.g. show that nonzero interplanetary magnetic field (IMF) By conditions lead to a twisted magnetotail configuration. The entire tail plasma sheet is rotated around its axis and tail magnetic field lines are twisted from the north–south symmetry: an additional By term collinear to the IMF By appears in the tail , with higher values in the plasma sheet than the lobes and the maximum close to the neutral sheet e.g..

The twisting is expected to be closely associated with plasma convection in the nightside geospace. Observations of nightside ionospheric convection have shown that fast transient azimuthal flow bursts are regularly seen in the midnight sector specifically during northward and By-dominating IMF and nonsubstorm conditions e.g.. These flow bursts have been suggested to be a manifestation of the untwisting process of the twisted tail field lines during the magnetic field reconfiguration following reconnection in a twisted tail .

According to the tail untwisting hypothesis by , tail twisting introduced by a prolonged exposure to significant IMF By causes the ionospheric footprints of a reconnecting tail field line to be azimuthally displaced in the Northern and Southern Hemispheres. Consequently, the newly reconnected field line will have to travel faster in one hemisphere to convect, via dusk or dawn, back towards dayside. This gives rise to the fast azimuthal flow bursts observed in the nightside ionosphere. The situation of the untwisting of magnetic field lines in a positively twisted tail (the field lines twisted counter-clockwise and the neutral sheet rotated clockwise around the Sun–Earth line when viewed from downtail looking Earthward) according to the hypothesis is illustrated in Fig. 1.

Interhemispheric observations of ionospheric azimuthal flow bursts support the above idea of untwisting and have confirmed that the asymmetric convection patterns in the Northern and Southern Hemispheres are resembling mirror images of each other . In addition, have presented simultaneous in situ magnetospheric observations made when these flow bursts were seen in the ionosphere. The observations showed that these ionospheric azimuthal flow bursts were associated with Earthward fast flows, which had a convective azimuthal velocity component matching the ionospheric measurements.

Recently, motivated by the results by , using Cluster data conducted a statistical investigation of a possible IMF dependence (and hence twist-dependence) of the direction of the azimuthal convection velocity component of the Earthward magnetotail fast flows. It was found that the azimuthal flow direction of the fast flows and the orientation of the IMF show a correlation and in such a manner that is consistent with the untwisting hypothesis. Surprisingly, the correlation was found to be quite strong regardless of the IMF Bz direction. One could expect stronger correlation for northward IMF conditions, since ionospheric azimuthal flow bursts are observed mainly during northward and By dominating IMF. For southward IMF, clearly azimuthally oriented ionospheric flow bursts in the midnight sector are much less frequent . Also, it has been shown both observationally and in simulations e.g. that the twist of the tail plasma sheet around its axis is considerably stronger for northward than for southward IMF. Whether this is true even for the degree of magnetic field line twisting remains to be shown. One possible interpretation of the results by is that the untwisting process could be occurring in the tail to some extent for all nonzero IMF By conditions, and not only limited to those of northward and By dominating IMF. This possibility should be studied further.

The untwisting hypothesis predicts a generation of an azimuthal flow shear by the Earthward transported untwisting magnetic field lines. Figure 2 illustrates the situation for positive tail twisting. In the figure the black square indicates a spacecraft that would observe an azimuthal velocity reversal when moving in the north–south direction across the neutral sheet. According to the hypothesis, the flow reversal is expected to be located at or close to the neutral sheet for untwisting field lines near local midnight. Farther away from midnight, i.e. towards the dawn and dusk flanks of the region of untwisting field lines, the flow reversal is predicted to be situated above or below the neutral sheet: the division between dawnward and duskward flows is the interface between the dawnward and duskward moving field lines, which is inclined relative to the tilted neutral sheet. Figure 1 shows the situation for positive tail twisting, in the case where the flow reversal is located below and above the neutral sheet dawnward and duskward of the centremost untwisting field lines, respectively (see also , their Fig. 6c). Note that in such a situation it is expected that the tail field lines are twisted significantly out of a meridian plane and have a fairly small inclination angle to the neutral sheet.

However, there is a lack of reported observations of the flow shear and the untwisting process in general in the magnetosphere. Some indications of a flow shear have been suggested to be observed by . Their observations from either side of the tail neutral sheet showed convective flows with opposite azimuthal sense in the opposite sides of the neutral sheet, which suggests closed field lines twisted significantly out of a meridian plane. In addition to that case, which occurred during northward and By-dominating IMF conditions, have reported an azimuthal flow shear across the neutral sheet in association with an Earthward fast flow. This event occurred during southward and By-dominating IMF conditions. suggested that the flow shear could be related to untwisting, but could not rule out a possibility of it to be related to internal flow patterns within the fast flow channel (depleted bubble), since the flow shear was observed only by one spacecraft. Besides these two studies, also have reported an indication of an azimuthal velocity shear across the neutral sheet during a fast flow in their fast flow example event. However, they showed only single satellite measurements and did not discuss the event any further. It is clear that detailed observations about these fast flow-associated azimuthal flow shears with reversals are needed to understand their nature and how they are related to a magnetotail fast flow and possible untwisting of closed field lines in the magnetotail.

In this paper, we analyse and discuss the flow shear event identified by in more detail. We will extend their preliminary single spacecraft analysis by taking the full advantage of the Cluster multispacecraft observations. In addition, ionospheric convection measurements by the SuperDARN network are used for analysing the Cluster observations in terms of the untwisting hypothesis. The paper is organized as follows: the instrumentation used is described in Sect. 2, the flow shear event is presented in Sect. 3 and discussed in Sect. 4, and the summary of the results together with conclusions are given in Sect. 5.

The magnetospheric observations for this study are made by the Cluster satellites (C1–C4). We use spin-resolution (∼ 4 s) velocity data from the Hot Ion Analyser (HIA) detector (C1 and C3) and from the COmposition DIstribution Function (CODIF) detector (C4) of the Cluster Ion Spectrometry (CIS) instrument . In addition, 3-spin resolution (∼ 12 s) omni-directional ion energy distribution data in particle energy flux units from the C1 CIS HIA detector are presented. Magnetic field data, as well as spin resolution, come from the FluxGate Magnetometer experiment (FGM) . Since the C2 spacecraft has no operational CIS instrument onboard, we do not show any data from the C2 satellite. IMF data presented in this study are 1 min ACE satellite measurements propagated to the bow shock nose provided by the OMNI database (http://omniweb.gsfc.nasa.gov/). The IMF and magnetospheric data are presented in the geocentric solar magnetic (GSM) coordinates throughout the study if not stated otherwise.

The ionospheric observations used in this study are 2 min large-scale ionospheric convection maps provided by SuperDARN . SuperDARN is an international network of HF radars located around the polar regions of both hemispheres. The radars provide continuous monitoring of the large-scale horizontal ionospheric plasma flow in the polar ionospheres and their data are hence suitable for comparison with flow observations from Cluster. The SuperDARN convection maps are derived by fitting a solution for the electrostatic potential, which is expressed in spherical harmonics, to the line-of-sight velocity measurements (“Map Potential” model; ). The equipotentials of the solution represent the plasma streamlines of the modelled convection pattern. Where no measurement points are available, information from the statistical model of , parameterized by concurrent IMF conditions, is used.

The idea of tail untwisting has been suggested by to explain the observations of fast azimuthal flow bursts seen in ionospheric convection in the midnight sector during predominantly northward and By dominating IMF and nonsubstorm conditions. However, there has been a lack of magnetospheric observations that show the hypothesized untwisting process in action. Only the case study by and to some extent also by show magnetospheric measurements that could be related to or explained by the untwisting phenomenon. report two fast flow events in which the convective azimuthal components match the conjugate ionospheric flows, which is in agreement with the untwisting hypothesis. In particular in their second event, present observations of opposite azimuthal convection by two Cluster satellites from either side of the tail neutral sheet, which suggests an existence of an azimuthal flow shear across the neutral sheet. report an azimuthal flow shear across the neutral sheet observed by only one of the four Cluster satellites in association with an Earthward fast flow, but they are not able to conclude whether the flow shear is related to tail untwisting or not, and do not discuss it any further.

The detailed multispacecraft magnetospheric observations presented in this paper confirm the previous indications of a possibility of an existence of an azimuthal flow shear across the neutral sheet within a fast flow and provide strong supportive evidence for the tail untwisting hypothesis: first, the Cluster observations show a positive tail By component collinear with the concurrent IMF By direction for several hours before the event. The ion data suggest that the collinearity occurs both on open and closed field lines, which proposes positive twisting of the tail magnetic field lines. In addition, the T96 model produces positively twisted closed field lines and the TF04 model an indication of a positively tilted neutral sheet for the event, which are in agreement with the Cluster observations. The actual plasma flow signatures within the fast flow show an azimuthal flow shear across the neutral sheet in the direction that would be expected in untwisting of positively twisted field lines. When comparing the flow shear with the conjugate ionospheric flows by SuperDARN we find that the directions of the azimuthal flow component in the opposite sides of the neutral sheet match to the flow directions of the ionospheric flows in the corresponding hemispheres. Furthermore, the ionospheric flow patterns resemble the flow patterns that have been previously attributed to the occurrence of untwisting e.g.. The azimuthal directions within the fast flow are also consistent with the statistical results of of IMF-dependent favourable azimuthal flow directions of convective fast flows, which they suggested possibly to be originated from untwisting.

The analysis of the flow shear reveals a small offset between the neutral sheet and the location at which the azimuthal flow reverses. This is the first time such an observation is reported. The flow reversal or the change of the V⟂y sign occurs slightly below the neutral sheet with an offset of Bx∼-0.4 nT. The untwisting hypothesis predicts an existence of such kind of offset. According to the hypothesis, the azimuthal flow reversal generated by the Earthward transported untwisting magnetic field lines is expected to be located at or close to the neutral sheet for untwisting field lines near local midnight. Farther away from midnight, i.e. towards the dawn and dusk flanks of the region of untwisting field lines, the flow reversal should be situated above or below the neutral sheet. Whether the reversal should be located below the neutral sheet in the dawnside and above in the duskside, or vice versa, depends on whether the tail is twisted positively or negatively, respectively. In the event studied in this paper, the flow reversal is located below the neutral sheet, which for positive tail twisting suggests a possibility that the neutral sheet crossing occurs in the dawnward part of the channel of the Earthward fast flow.

However, the ∼-0.4 nT Bx offset is small and its magnitude is comparable with (in fact smaller than) the standard deviation ∼0.5 nT derived from the regression analysis (red line in Fig. 7). Hence, one may argue that deducing the Cluster neutral sheet crossing region using the offset in the Bx component would not be justified. On the other hand, there is a possibility that the plasma frame in the vicinity of the spacecraft could be moving dawnward at V⟂y∼-24 km s-1, which would cause the offset.

The question also arises, whether the observed flow shear and azimuthally oppositely directed flows can be explained by any other mechanism. suggested their event to be related to untwisting, but also discussed the possibility that the flows could be related to flow patterns internal to the fast flow channel (depleted bubble). They lacked SuperDARN ionospheric flow observations from both hemispheres and thus could not draw any definite conclusions regarding the cause for the shear in the flows. , on the other hand, had supportive ionospheric data available and concluded that the oppositely directed azimuthal flows in the magnetosphere in their event were due to untwisting. Fortunately, for our event, SuperDARN observations are available in both hemispheres, and they together with the Cluster observations give support to the untwisting scenario. The results of this study further suggest that the oppositely directed azimuthal flows above and below the neutral sheet are a global property of the channel of the Earthward fast flow and not any local disturbance within the flow pattern.

In this study, we have presented detailed multispacecraft observations made by Cluster of an azimuthal velocity shear in the perpendicular flow component within an Earthward fast flow in the magnetotail. The fast flow event, which occurred on 5 September 2001, showed a clear flow shear velocity pattern, i.e. a V⟂y sign change, around the neutral sheet (Bx∼0) associated with a fast flow during the neutral sheet flapping motion across the satellite locations. This suggests that the convection within the proper flow channel of a fast flow may not generally be azimuthally unidirectional across the neutral sheet, but may have a more complex structure.

The event was associated with northward and By dominating IMF conditions without any signatures of substorm activity. However, the magnetospheric data measured by Cluster showed some signatures that could possibly be related to a twisting-associated stretching of the magnetotail before the event, analogously to a substorm growth phase. The observed fast flow would then be a natural manifestation of a localized tail relaxation process. Generally, both the Cluster observations and the T96 model suggest a twisting of closed field lines in the positive direction (positive By collinear with the positive IMF By). In addition, the TF04 model indicates weak but consistent tilt of the neutral sheet.

During the fast flow, all the three Cluster satellites (C1, C3 and C4) observed a change of the azimuthal velocity component from dawnward to duskward when moving from northern field lines to southern field lines as the neutral sheet moved in the +ZGSM direction over the satellites. The directions of the azimuthal flows at both sides of the neutral sheet are in agreement with that which would be expected for the flow directions if the flows were directed due to untwisting during the magnetic field reconfiguration following reconnection in a twisted tail e.g..

The flow reversal i.e. V⟂y sign change did not occur precisely at the neutral sheet but was displaced somewhat below it. At the reversal the Bx component had a value of ∼-0.4 nT. In addition, an indication of a linear dependence between the Bx and V⟂y was found close to the neutral sheet (-2 ≤ Bx ≤ 2 nT). A linear fit gave a relation V⟂y=-53.8⋅Bx-23.9, where V⟂y and Bx are given in km s-1 and nT, respectively.

This observation of an offset between the neutral sheet and the flow reversal is the first reported in the literature. However, the untwisting hypothesis predicts an existence of such kind of offset. Towards the dawn and dusk flanks of the flow channel the flow reversal is expected to be situated above or below the neutral sheet depending on the tail twisting direction. The observations provide us with the opportunity to infer the neutral sheet crossing region in respect to the dawn–dusk extent of the fast flow channel in the framework of the untwisting hypothesis. In this event, the neutral sheet crossing was inferred to occur in the dawnward part of the channel of the Earthward fast flow. However, we cannot rule out uncertainties associated with the offset (smaller than the standard deviation) nor other explanations like the possibility of motion of the plasma frame in the vicinity of the spacecraft as factors that could affect the correctness of our inference. The linear dependence between the Bx and V⟂y close to the neutral sheet within the fast flow is also an interesting new finding and it should be investigated whether this relation is a typical feature for fast flows during nonzero IMF By conditions.

During the event, ionospheric convection observed by the SuperDARN radars showed enhanced eastward plasma flows in the Northern Hemisphere and plausible westward flows in the Southern Hemisphere in the conjugate regions. This is consistent with the azimuthal flow directions measured by Cluster. However, obviously, there exists a problem with proper mapping of the Cluster measurements into the ionosphere. In this event, the T96 mapping provided a clearly too small azimuthal displacement for mapped footprints due to T96 model underestimation of the true twisting in the magnetotail. However, the ionospheric observations resemble those flow patterns previously attributed to the untwisting of twisted tail field lines e.g. supporting the possibility that the flows measured by Cluster were due to untwisting. Hence, by combining all evidence provided by the data presented here, we suggest that the fast flows observed in the magnetosphere were a manifestation of the untwisting of twisted tail field lines, although the proper ionospheric mapping of magnetospheric observations from a twisted tail remains a problem. Thus, fully consistent conclusions cannot be drawn.

In conclusion, the event presented in this paper provides strong supportive evidence for the untwisting hypothesis which explains the occurrence of azimuthally directed flows in the nightside ionosphere and magnetosphere by untwisting of twisted magnetic field lines during nonzero IMF By conditions. Albeit our observations show a clear multispacecraft example of an azimuthal flow shear across the neutral sheet within a fast flow, more observations are needed to establish whether the flow shears with reversals are an essential feature of the structure of the fast flows.