Understanding the mechanisms that cause the range of different near-Earth space phenomena is often a difficult task. The system is highly coupled, from the thermosphere/ionosphere to the solar wind. It can therefore be challenging to determine the underlying cause of observed phenomena, as multiple processes occurring in different regions, alone or in combination, may produce very similar observational signatures.

This paper addresses why we observe a different behaviour of the magnetosphere-ionosphere system when the B_y component of the Interplanetary Magnetic Field (IMF) is positive vs negative during periods when the Earth’s magnetic axis is tilted towards or away from the Sun. Although first mentioned by Friis-Christensen and Wilhjelm (1975), it was not until recently that this topic has been revisited and further characterized. Friis-Christensen and Wilhjelm noted that during northern hemisphere winter (negative dipole tilt angle, Ψ), the westward electrojet was significantly stronger for positive compared to negative IMF B_y during otherwise similar IMF B_z conditions. Holappa and Mursula (2018) revisited this effect on the westward electrojet and established that these differences were not due to the Russell-McPherron effect (Russell and McPherron, 1973). They called this the explicit B _y effect to differentiate it from the Russell-McPherron effect, which is essentially due to seasonally varying correlation between IMF B_y and the geoeffective B_z component (IMF B_y not directly causing the Russell-McPherron effect). Holappa and Mursula (2018) quantified the difference in electrojet strength to be about 50 percent during winter conditions. During summer conditions, the IMF B_y dependence reverses as the westward electrojet is stronger for negative compared to positive IMF B_y, but the effect on the westward electrojet is minor compared to the difference during winter conditions (Holappa and Mursula, 2018; Holappa et al., 2021b).

The large IMF B_y and seasonal related asymmetries in the westward electrojet sparked the interest for investigating other aspects of the coupled system for similar behavior. Reistad et al. (2020) reported similar asymmetries in the average size of the polar cap. During negative dipole tilt, they found larger polar caps in both hemispheres when IMF B_y is positive compared to negative. The ± B_y asymmetry reversed when the dipole was tilted in the opposite direction. Since the same behaviour was seen in both the winter and summer hemisphere for a specific IMF B_y polarity, ionospheric effects related to season could not alone explain the observations. They suggested that in addition, one or both of the following scenarios must be the case: A) The global dayside reconnection rate depends on IMF B_y polarity when Earth’s dipole is tilted, allowing for a stronger Dungey cycle and more energy input to the system when IMF B_y and Ψ have opposite signs; or B) When Earth’s dipole is tilted, IMF B_y has an influence on the amount of magnetic flux the magentotail lobes typically can support, for a given dayside reconnection rate. While the results reported by Reistad et al. (2020) indicate a global difference in the magnetospheric response, the minor effect on the westward electrojet in the summer hemisphere compared to the winter hemisphere suggests that the influence on the ionospheric currents are more complex than what can be explained by type A and B mechanisms.

Other aspects of the solar wind - magnetosphere coupling has also been investigated in this regard. Holappa et al. (2020) and Holappa and Buzulukova (2022) found that fluxes of precipitating energetic electrons and protons in both hemispheres show a very similar asymmetry, namely that the precipitation is more intense when IMF B_y and Ψ have opposite signs, compared to when they have the same sign, during otherwise similar IMF B_z conditions. Holappa and Buzulukova (2022) showed that the growth rate of the ring current (measured by the Dst index) also exhibits a similar B_y-dependence. Ohma et al. (2021) investigated the occurrence frequency of magnetospheric substorms during periods of either positive or negative IMF B_y. Based on several independent lists of identified substorms, they concluded that substorms are more frequent when the sign of IMF B_y and Ψ are opposite, compared to when IMF B_y has the same sign as the dipole tilt. These findings further indicate that the explicit B_y-effect is a global phenomenon, which is likely related to a type A or B mechanism (or both) as mentioned above. Since dayside and nightside reconnection are the processes that allow for opening and closure of magnetic flux; the steady state, in which we interpret the long-term averages to represent (Laundal et al., 2020), must represent a balance of the two. The observed changes of the steady state open flux content depending on IMF B_y/Ψ polarity must therefore either be due to a type A or B mechanism (or both). These spatially separated processes (A: dayside and B: tail) are nevertheless highly coupled, as the dayside loading affects the conditions in the magnetotail, making the interpretation of the data analysis highly challenging, as will be elaborated on in more detail in the following.

Reistad et al. (2021) studied ionospheric convection on the basis of Doppler-shift from ground based HF radar echoes. They presented climatological patterns of the high latitude convection pattern during IMF B_y dominated periods, for various dipole tilt intervals. By normalizing the observed convection to the present size of the polar cap as inferred from simultaneous observations from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE), they were able to distinguish the contribution from lobe reconnection on the global convection pattern, which were found to be more efficient during local summer. This allowed the authors to quantify the part of the convection associated with dayside (and nightside) reconnection. Their results indicated that when IMF B_y and dipole tilt had opposite polarity, the Dungey cycle was slightly enhanced ( ∼ 10 % ) .

The results presented above demonstrate that the global magnetospheric response to positive and negative IMF B_y is different when Ψ is nonzero: The westward electrojet is stronger, particle precipitation is more intense, substorms are more frequent, the polar cap is larger, and the ring current increases more rapidly when the signs of IMF B_y and Ψ are opposite. Previous studies have not identified whether this is mainly due to differences in the dayside coupling (type A) or mainly due to differences in how the tail responds to the same flux loading, influencing the amount of magnetic flux the lobes typically sustain (type B). However, there are key differences in how the magnetosphere and ionosphere are expected to respond between the type A and type B mechanisms, enabling new insights to be obtained about their relative importance.

If the observed explicit B_y effects are solely a result of mechanism B, the flux throughput in the (tilted) magnetosphere should be the same for both polarities of IMF B_y, if all other solar wind parameters are equal. Since substorms are more frequent for opposite signs of IMF B_y and Ψ, the substorms must either be stronger (closing more flux) or the flux must be transported by another process when IMF B_y and Ψ have the same sign. In addition to substorms, the magnetosphere can respond to solar wind forcing by entering into steady magnetospheric convection (SMC) periods (e.g., Sergeev et al., 1996; Kissinger et al., 2012). Hence, a type B process would imply weaker substorms and/or less frequent SMC events for opposite signs of IMF B_y and Ψ. On the other hand, mechanism A demands the flux throughput to be greater in the magnetospheric system for positive B_y during Northern Hemisphere (NH) winter and for negative B_y during NH summer. If the occurrence frequency of SMCs and the strength of substorms follow the same dependence as the other observed phenomena (stronger for opposite signs of IMF B_y and Ψ), it would suggest the influence of a type A mechanism in producing the observed explicit B_y effects.

In this paper, we present new analyses to demonstrate how IMF B_y and Ψ in combination affects the response of the magnetosphere-ionosphere system. The goal is to address whether mechanisms A or B are the main contributor to the explicit B_y effects. We will address how the occurrence frequency of SMC events are modulated and use various proxies to assess the global substorm strength. In the following section we will present our new analysis contributing to the investigation of the origin of the explicit B_y effect. The methodology used in the presented analysis is outlined as the results are presented. Our new results are discussed together with the existing knowledge on the topic in section 3. Our main conclusions are explicitly stated in the concluding section.

When comparing the magnetosphere responses to intervals of positive and negative IMF B_y and dipole tilt, it is important that we compare instances with similar dayside forcing. This is largely controlled by the IMF B_z component. In all our analyses, we use the dayside coupling function presented by Milan et al. (2012), which is set out to quantify the global dayside reconnection rate, Φ _D [Wb/s], in response to the upstream IMF and solar wind, given by the formula Φ D = Λ V x 4 / 3 B yz ⁡ sin 9 / 2 θ / 2 . (1)

Here, Λ = 3.3 ⋅ 10⁵ m^{2 /3}s^1/3 is a scaling constant related to the length of the dayside X-line, V _x is the solar wind velocity in the anti-sunward direction, B _yz is the magnitude of the IMF vector perpendicular to the Sun-Earth line, and θ is the IMF clock angle. When SI units are used, Φ _D gets the units of Wb/s = Volt, representing the rate of conversion of magnetic flux from a closed to open topology on the dayside.

As noted by Holappa et al. (2021a), the particular choice of coupling function is typically not crucial for this kind of analyses, yet they provide a convenient way to select periods of similar forcing for comparison. In all plots, the dayside coupling value is normalized to its mean value from the entire analysis period. This normalized coupling parameter we denote Φ _D /⟨Φ _D ⟩. We use IMF data time shifted to the bow shock of the Earth from the OMNI database (King and Papitashvili, 2005).

In recent years, a growing body of evidence (see introduction) has demonstrated how various aspects of the solar wind—magnetosphere—ionosphere interactions depend on the combination of the polarities of IMF B_y and dipole tilt, often referred to as an explicit B _y effect. That is, for a given, nonzero tilt angle Ψ, the system responds differently to positive and negative IMF B_y. In this paper we provide further observational evidence of this effect, aiming to provide observational constraints on the source of the explicit B_y behaviour of the system. Two types of mechanisms have been suggested to explain the explicit B_y effect:

A) The combination of the polarity of IMF B_y and dipole tilt affects the global dayside reconnection rate, with higher flux throughput when the two has opposite polarity compared to equal polarity.

Observations consistent with a type B mechanism have been reported on earlier (Holappa et al., 2021a). However, can this type of process, confined to the magnetotail, be the sole explanation for the presented observations? This is the core question we address in this article. Evidence on the existence of a type A mechanism has not been presented to date (to our knowledge). There is an ongoing debate whether the dipole tilt angle modulates the dayside reconnection rate (Cliver et al., 2000; Russell et al., 2003; Lockwood et al., 2020). However, these studies have not addressed the combined action of the dipole tilt and IMF By (which are both included in the type A mechanism). Previous studies focusing on the explicit B _y effect have not been able to make definite conclusions about the relative importance of A versus B type of mechanisms. However, a critical test for the above hypotheses is to determine whether or not the magnetic flux throughput of the system (the strength of the Dungey cycle) is stronger for the opposite signs of Ψ and IMF B_y. This is predicted by mechanism A, but not by mechanism B. By combining the results presented in this study with earlier observations, we can now address how the flux throughput is modulated by IMF B_y and Ψ.

Milan et al. (2008) investigated how the magnetosphere responds to weak and strong solar wind forcing, and showed that enhanced magnetic flux input (Φ _D ) lead to both stronger and more frequent substorms. It has also been shown that the substorm occurrence frequency is higher when IMF B_y and Ψ have opposite compared to equal polarity (Ohma et al., 2021). The global substorm strength shown in Figure 2 demonstrates that the strength of substorms is also greater, therefore closing more magnetic flux per substorm (Milan et al., 2009), on average, when signs of IMF B_y and tilt angle are opposite compared to equal. Taken together, these results strongly suggests that the IMF B_y/Ψ polarity combination affect the magnetic flux throughput, hence supporting a type A mechanism.

The IB latitude analysis presented in Figure 4, which can be interpreted as the transition region from dipolar to stretched field lines, is systematically displaced based on IMF B_y and dipole tilt polarity, highlighting the profound influence by these parameters. This region of the magnetotail is known to depend upon the previous magnetospheric activity (and hence solar wind - magnetosphere coupling) with a significant memory. Newell et al. (2007) found the IB latitude to show the best correlation with a 6 h long averaging window of the upstream forcing. It is therefore highly expected that a type A mechanism would lead to the observed differences in Figure 4. The observed shift in IB latitude is remarkably similar to the corresponding shift in polar cap radius during similar conditions, as reported by Reistad et al. (2020). However, as Reistad et al. (2020) pointed out, we can not exclude a type B mechanism influencing these results, when interpreting this analysis alone. However, when interpreting these results in light of our findings regarding the substorm strength as discussed above, the we find that a type A mechanism is the most plausible scenario to explain also the IMF B_y polarity effect on IB latitude.

Interpreting the onset latitude as a metric of the strength of the substorm in terms of flux closure (Milan et al., 2009), the similar onset latitude for ± B_y (Figure 3) combined with their more frequent occurrence (Ohma et al., 2021) suggests a larger flux throughput when IMF B_y and Ψ has opposite polarity, in agreement with a type A mechanism. We find no direct evidence that the substorm onset shifts to higher latitudes for opposite IMF B_y and tilt polarity compared to same polarity, as would be indicative of a type B mechanism (the more frequent substorms closing less flux each, hence taking place at a higher latitude [e.g., Milan et al. (2009)] to accommodate the same flux throughput). This analysis is therefore consistent with the conclusions above, namely that a type A mechanism is likely to exist during these conditions. One may argue that if the global dayside reconnection rate is affected (type A mechanism) as suggested, a lower onset latitude would be expected for opposite B_y and Ψ polarity, contrary to what we see (the weak trend in the suggested direction is below the level of statistical significance). One possibility is that both a type A and B mechanism may be present, since their influence on onset latitude is expected to be opposite. If so, the presence of a type B mechanism indicates inherent limitations of addressing the strength of a substorm only through its onset latitude. Nevertheless, our main conclusion remains, namely, that a type B mechanism alone can not explain the results, and that the global dayside reconnection rate has an explicit B_y dependence during periods of significant dipole tilt.

As mentioned in the introduction, more frequent SMC intervals during the IMF B_y/tilt polarity associated with more frequent substorms would be indicative of a type A mechanism. This is indeed what we see in Figure 1. It is nevertheless relevant to mention the study by Milan et al. (2019) in this regard, showing that the substorm onset latitude is an important parameter in determining whether the magnetotail develop into a period of SMC if the IMF remains southward after a substorm. They found that substorms taking place in an extended oval (below 65° MLAT) was less likely to develop into an SMC interval, and suggested this was due to the atmosphere invoking friction on the ionosphere-magnetosphere system, since lower latitude substorms are typically stronger. From Figure 2 we have seen that the more frequent substorms observed for opposite B_y/tilt polarity (Ohma et al., 2021) are associated with stronger substorms. The fact that we also observe more frequent SMC intervals when IMF B_y and the dipole tilt have opposite signs suggests that the effect reported on by Milan et al. (2019) is less relevant for the more typical substorms considered here. In fact, the quartile binning used in Figure 3 shows that three of four bins (75% of the onsets) have a median onset latitude above 65°, which was the limit for “convection breaking” used by Milan et al. (2019).

For opposite compared to equal polarity of IMF B_y and tilt angle, observations show stronger substorms, more frequent substorms and more frequent SMCs. Furthermore, the average IB latitude and polar cap radius indicate a larger oval, which means that the average flux content is also greater for opposite IMF B_y and Ψ. In addition, Reistad et al. (2021) found 10% stronger flux throughput during the same conditions. Taken together, we conclude that the body of evidence presented is pointing towards mechanism A being the main source of the observed explicit B_y behaviour of the system. However, we can not exclude that a type B mechanism take place at the same time, but taken alone, a type B mechanism is insufficient to explain the observed behavior.

The results from this paper suggests that an explicit B_y effect should be included in future global dayside reconnection rate coupling functions, which is expected to enhance the predictive abilities of geospace activity. When taking this contribution into account, it would likely be easier to make further constraints on the importance of mechanism B type of processes.

Although we suggest that an explicit B_y effect is present on the global dayside reconnection rate, we have at present no good understanding of why. As suggested earlier by Reistad et al. (2020) and Ohma et al. (2021), the many dawn-dusk asymmetries upstream of the magnetopause (Walsh et al., 2014) may introduce dawn-dusk asymmetries in the local dayside reconnection rate. How this combine with a tilted dipole will be a very interesting topic to explore with 3D global kinetic models such as the Vlasiator model (Palmroth et al., 2018).

Based on the new analysis presented in section 2, together with recent advances in describing the geospace response during these conditions (Holappa and Mursula, 2018; Holappa et al., 2020; Reistad et al., 2020; Holappa et al., 2021b; Ohma et al., 2021), we conclude that the global dayside reconnection rate is likely to be enhanced when IMF B_y and the dipole tilt have opposite signs (± and −/+), compared to when they have the same signs (−/− and +/+). This is referred to as a type A mechanism. We have also discussed the possible contribution from a type B mechanism, where the magnetotail response may depend on IMF B_y and the dipole tilt, affecting the amount of magnetic flux the magnetotail lobes typically can support. While we can not neglect that a type B mechanism has a significant contribution to the observed response, we find it insufficient to explain our analysis alone, pointing toward the existence of a type A mechanism taking place at the dayside of the magnetosphere. The detailed physical mechanism of such an effect should be further investigated.