The high-latitude atmosphere is a dynamic region with processes that respond to forcing from the Sun, magnetosphere, neutral atmosphere, and ionosphere. The dominant coupling occurs between the ionosphere, magnetosphere, and the solar wind. Interactions between the interplanetary magnetic field (IMF), the magnetic field carried by the solar wind, and the terrestrial magnetosphere result in magnetic reconnection. This creates an area of open field lines (field lines that originate at Earth and connect to the IMF) known as the polar cap. The physical processes that occur here are different from those that occur at other high-latitude regions where the magnetic field lines are closed (connect back to the Earth in the opposite hemisphere). In the polar cap, magnetic field lines are moved from magnetic noon to magnetic midnight by the solar wind, where they eventually reconnect with geomagnetic field lines from the opposite hemisphere. Once closed, these field lines move to lower magnetic latitudes (the auroral oval) and return towards the dayside. This process of reconnection is known as the Dungey cycle , and (to first order) describes the motion of the magnetic field lines and the ionospheric plasma frozen into those field lines.

At ionospheric altitudes, the open–closed field line boundary (OCB) separates the polar cap from the auroral oval, which is the highest latitude region to have closed magnetic field lines. This boundary is important because the state of the field lines (open or closed) determines the types of coupling that may occur within the magnetosphere–ionosphere–thermosphere (MIT) system. One example of a difference in MIT coupling between the polar cap and auroral oval is field-aligned currents (FACs). The closed field lines in the auroral oval support the formation of current systems that link the ionosphere to the magnetopause and current sheet (the Region 1 or R1 FAC system) and to the partial ring current in the inner magnetosphere (the Region 2 or R2 FAC system) . Because the R1 FAC system connects the ionosphere to the outer magnetosphere, it lies poleward of the R2 FAC system and moves with the OCB and references therein.

Another example of MIT coupling processes affected by the OCB is the density structure of the high-latitude ionosphere. Consider the unexceptional case of a southward IMF and a partially illuminated high-latitude ionosphere. Under these conditions, ionospheric plasma follows a convective flow driven by the Dungey cycle, characterised by straight, anti-sunward plasma drifts within the polar cap and longer, curved, sunward drifts when the plasma are frozen into closed magnetic field lines (the boundary between these two regions is commonly referred to as the convection reversal boundary or CRB). The difference in convective motion poleward and equatorward of the CRB creates a highly structured polar ionosphere, as the dense dayside ionospheric plasma is rapidly transported to the nightside where recombination processes destroy plasma that does not return to sunlit regions quickly enough (due to having to follow the longer return path through the auroral oval) e.g..

Due to these and other differences in MIT coupling processes in the auroral oval and the polar cap, it is desirable to have a coordinate system that indicates where (in which region) measurements were taken. This type of adaptive, high-latitude gridding has been performed with various data sets . These studies have demonstrated improved statistical and climatological results (for example, demonstrated the difference between using magnetic and OCB-oriented coordinates when studying the climatological behaviour of the plasma drift vorticity) when using adaptive, high-latitude coordinates. Unfortunately, observations of the OCB are sparse. Long-term and large-scale studies would benefit from specifications of the OCB in both hemispheres and all magnetic local times (MLTs) every 15 min or less . Models that have the ability to distinguish between regions with open and closed field lines would also benefit from adaptive, high-latitude coordinates .

This study presents a new set of OCBs obtained from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) magnetic perturbation observations. AMPERE measurements of FACs make it possible to estimate the location where Region 1 (R1) and Region 2 (R2) FAC systems meet (the R1–R2 boundary). Because the location of the Birkeland current system is tied to the expansion and contraction of the polar cap under quiescent and disturbed conditions and references therein., it seems logical to hypothesise that a dependable relationship between the R1–R2 boundary and the OCB exists. This study investigates the relationship between the AMPERE R1–R2 boundary and the OCB inferred from particle precipitation measurements made by the Defense Meteorological Satellite Program Special Sensor J (DMSP SSJ) electron energy flux boundaries. This study has parallels with that of , who compared the R1 peak location (as determined from a circle fitted to the R1 peaks at all MLTs) with a range of different DMSP particle precipitation boundaries, showing a close relationship with the b5i and b5e boundaries in the nightside ionosphere. Section  presents the details of both data sets. Section  explores the relationship between the different boundaries and presents the calibration process that allows the AMPERE R1–R2 boundary to be used as a proxy for the OCB. This calibration, as well as the previous Magnetopause-to-Aurora Global Exploration (IMAGE) calibration performed by , is validated in Sect.  by comparing calibrated OCBs with the CRBs from DMSP plasma drift measurements. CRBs were chosen as a validation data set because the direction of convective plasma drifts are strongly tied to the motion and state (i.e. open or closed) of the magnetic field lines. This means that the CRB is typically located at or just equatorward of the OCB , except for regions of the dayside and nightside ionosphere that map to regions of ongoing magnetic reconnection. Finally, the results of this study are summarised in Sect. .

The data sets used in this study have a long and ongoing history of observations. The primary data set, AMPERE, is described in Sect. . Two instruments from DMSP are used, one for calibration of the boundaries and another for validation. Both DMSP data sets are described in Sect. . The IMAGE far ultraviolet (FUV) data set used in the validation is described in Sect. .

This study follows the process outlined in , which determined the offset between the IMAGE FUV poleward auroral boundaries and DMSP OCBs, to obtain a correction between the AMPERE R1–R2 boundary and the DMSP SSJ OCBs. The five steps of this process are enumerated in the following:

identify the AMPERE R1–R2 boundaries;

The appropriateness of using K to transform the AMPERE R1–R2 boundary into an AMPERE OCB is tested by comparing the AMPERE OCBs to the DMSP CRBs within an hour of dawn and dusk. These local times were chosen due to the MLT-dependent variations in the CRB–OCB relationship discussed in Sect. . It should also be reiterated that no specific selection was made for IMF conditions. All IMF clock angles and magnitudes are considered together, as the AMPERE OCBs should be valid at all IMF conditions when the OCB can be represented (to first order) by an ellipse. To ensure that the performance of the AMPERE OCBs are on par with previous OCB calculations, this validation is also performed for the IMAGE OCBs. Unfortunately, it is impossible to directly compare the AMPERE and IMAGE OCBs because there is no temporal overlap between the two data sets. This validation effort paired OCBs with DMSP CRBs that were identified within 10 min of one another. The location of the DMSP CRB relative to the OCB was then determined. In this adaptive coordinate system, the OCB is set at a co-latitude of 74∘ (a latitude chosen to represent the OCB in adaptive, high-latitude coordinates based on the typical size of the polar cap). CRBs that occur poleward or equatorward of the OCB will have co-latitudes greater than or less than 74∘ respectively. This adaptive gridding was performed using the ocbpy Python package .

Figure  shows the distribution of CRB observations for the different DMSP satellites, OCB sources, and hemispheres. As was done with the DMSP SSJ observations, 2 years of CRBs and OCBs were paired in time after removing unreliable boundaries (as discussed in Sect. ). Note that the paired data, both from the two IMAGE instruments and from AMPERE (in both hemispheres), show a similar spread of CRBs at different magnetic local times, with larger spreads near magnetic noon and midnight.

Figure  shows the histograms of the latitude differences between the DMSP CRBs and the IMAGE (Fig. a, d) or AMPERE (Fig. b, c, e, f) OCBs. This figure also shows the results for the median ellipse correction to obtain the AMPERE OCB (Fig. 5a, b, c) and the S.G. peak ellipse correction (Fig. d, e, f). For the IMAGE histograms, Fig. a shows the results for the SI13 instrument, and Fig. d shows the results for the SI12 instrument. In all cases, the means and medians of the difference distributions behave similarly: most points lie within 1∘ of each other, and the standard deviation of the distributions is below 5∘ in all places. Additionally, the CRB is approximately co-located with both the AMPERE and IMAGE OCBs. This close agreement with the DMSP CRB and the similar behaviour of the IMAGE and AMPERE OCBs validates the AMPERE OCBs provided here.

This study modified traditional auroral boundary fitting methods to establish an MLT-dependent relationship between the OCB and the R1–R2 boundary. This was performed by determining the first moment of the distribution of differences between the R1–R2 boundary and the OCB (as measured by the DMSP SSJ instrument) for 1 h MLT bins. These moments (which included the median of the distribution and the peak of a smoothed Gaussian fit) were then used to define the parameters of an elliptical function. This function specifies the distance between the OCB and R1–R2 boundary as a function of MLT.

The validity of this OCB, as well as previously determined IMAGE OCBs, were tested against the dawn and dusk measurements of the CRB (as measured by several DMSP IVM instruments). These boundaries were found to typically differ by less than a degree.

As mentioned in the introduction, modelling and statistical studies in polar regions should avoid mixing measurements taken in the auroral oval and the polar cap. In combination, the AMPERE and IMAGE OCBs form the basis of a multi-solar cycle data set that could be used to improve high-latitude statistical studies and climatological models. The data sets and software tools presented in this paper allow researchers to begin using adaptive, high-latitude coordinates in their investigations.